404 Error on existing pages - asp.net

Have an issue where I get a 404 errors on multiple pages that definitely exist on the server.
It is an ASP based website and at completely random points I get a 404 error for those pages that definitely exist on the server as after I do a refresh it's absolutely fine.
Weird thing is that when I open developer tools I never receive the error anymore.
Any chance that developers tools is actually interfering with the connection between browser and server and actually changes something?
Why I'm not getting the error while developer tools is running?
Has anybody got anything similar to this before?
I am using Internet Explorer 11 as well as tried Internet Explorer 10.
Same on both browsers.

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IIS 500 error on postback or upload on some computers

I run a website on IIS 8.5 and ASP.NET. I recently moved my website to a new server. Some users are getting an internal server error when trying to postback or upload on the site. As an example, when they try to reply in the forum (I use YetAnotherForum), they will get the error page.
I think I've narrowed it down to postbacks. Whenever there is a postback, they will get the error. This includes using ASP.NET buttons on the page.
My big problem is that the error is not logged anywhere. It is not in the event viewer, failed request logs, or even the server log. I have all application errors caught and logged in Global.asax, but the error never reaches the handler. The users just get the error on the screen and it doesn't seem to be recognized at all by the server. Here is a screenshot of the error:
Generic IIS Error
As you can see, it is a very generic IIS error.
I am still maintaining a test site on my old server. I sent one person that was getting the error to the test site to see if he got the error. He did not - he only gets the error on the new site on the new server. I'm guessing it must be some setting I am missing.
Only a select few users are getting the error. The ones I have talked to are using Windows 10. They have tried all of the different browsers (IE, Firefox, and Chrome), and none of them work. I also have Windows 10, but I do not get the error.
Any chance someone has seen this error and knows the fix?

CRM 2013 Working fine then suddenly 404 error?

As the subject says I had a perfectly good CRM 2013 install on 2012 and it was being used. Sometime overnight the site went down. Now I get a 404.0 error. I've disbaled friendly errors...just get a blank page not even a 404. I've modified the web.config debug=true nothing. Fiddler shows nothing...well not exactly. Fiddler shows a 404 as well.
I've removed ssl cert and reapplied. Tried accessing the site http nada?? I looked for any updates that were pushed overnight nothing..(they are disabled but I checked anyway).
If I browse to http it redirects me https as it should. It prompts for my NT credentials as it should. Downloads favicon and decides it has done all it's going to do.
So I am at a real loss as to even getting a clue as to the problem. If someone can help me with some further troubleshooting direction it would be appreciated.
Thank You

Site functions locally, "Internet Explorer cannot display the webpage" on server

I have an ASP.Net Website (not a web app, fwiw) that builds and works just fine locally through IIS on my dev laptop.
However, when I publish it to our QA box and try to view while I'm remoting into that server, I get a message from IE saying "Internet Explorer cannot display the webpage". Firefox just spits back a quick "Connection Timed Out"
There is absolutely nothing in the event log nor the IIS log about this. I'm unsure where I can look for more info.
I'm fairly confident it is an ASP.Net issue. I can install a sample site from our vendor, Ektron, into IIS and it will run. If I overwrite the sample's web.config with my own, it continues to run. If I then blow away the entire sample site and copy over my site from my local, I'll get the message about how "Internet Explorer cannot display the webpage".
I've tried to keep the environments as close together as I can. Both boxes are running IIS 7.5 under an integrated app pool for .Net 4.0. I browse via localhost on dev and via an IP on the server.
I am not terribly familiar with the Website template, so I might be missing something obvious (I hope!). I'm hoping someone can provide some guidance into how I can get more info on what the heck is going on so I can resolve this issue.
UPDATE
I think I'm getting closer. By using Fiddler (thanks for the suggestion, Amy!) I notice that it redirects the request to SSL. SSL requires a different license from our vendor, so that might be it. I'm still trying to understand why that redirect is taking place, but at least I have something now to look at.
I'd look into the SSL settings in the web.config / IIS - settings for Mime Types and also into Ektron's MIME Types in the MimeType.config file. I found that some .aspx pages like (ekajaxtransform.aspx) weren't functioning correctly because of firewall/proxy issues/restrictions.
Hope it helps. :)

ASP.Net Forbidden URL with Internet Explorer

I have an issue here which has me baffled.
Another developer and myself developed an ASP.Net Web Forms application four years ago and the site has been live since. About 6 months ago we moved the application from a Windows 2003 web server using IIS 6, to a Windows 2008 web server using IIS 7.5. When we moved servers we tested the application again in the Firefox web browser, however, I can't remember if we also tested in Internet Explorer. We should have done.
Recently it has been brought to our attention that in Internet Explorer, when a user attempts to hit some of the pages within the website, they are presented with a
Forbidden URL
HTTP Error 403. The request URL is forbidden
I have also tried to recreate this error on my local computer and I also get an error
HTTP Error 400 - Bad Request
This only happens when using an Internet Explorer web browser, it works fine with the likes of Chrome, Firefox etc.
I am not sure how to fix this, but because myself and the other developer have not changed any of the source code, I don't think this is where the problem lies. I have a feeling it might have something to do with IIS on our new web server, but I am really not sure.
Has anyone else seen this problem before?
Any feedback would be greatly appreciated.
Thanks.
Folks, the problem for some reason was as follows:
When a user clicked a button on my site which would then redirect them to another page within the site, I previously had the following code to do so
Response.Redirect("../userprofile/myprofile.aspx")
However, after the release of IE 10, this redirect would no longer work, and instead I had to replace it with
Response.Redirect("~/userprofile/myprofile.aspx")
Hope this helps someone else who is having similar problems.

Posting forms to a custom 404 Classic ASP URL in IIS7: where is the post data?

Have recently migrated a number of websites from an ageing Windows 2003 server running IIS6 to a new Windows 2008 server running IIS7.
The sites all use a CMS written in classic ASP which requires that the 404 error is modified to load a default.asp file which looks at the URL and loads content from a database. This is just to keep all the URLs nice and SEO friendly.
The back office of the CMS works in a similar way (except the 404 error is dealt with by admin/default.asp).
For some reason though, the login page (which obviously involves a form posting to the custom 404 error) does not carry over the POST information.
OK - here's the weird bit. It works fine in Google Chrome, but not in IE7/8 or Firefox.
Obviously this is right royal pain in the harris.
Any ideas why post data is carried through the custom 404 handler for one browser but not any others?
I've got as far as I can and am currently trawling the web for information about this but can't seem to find anything...
OK - here's an update for the weird bit.
It works fine in Safari as well as Google Chrome - but not in IE7/8 or Firefox.
???
I'm not sure this helps, but check this question Posting forms to a 404 + HttpHandler in IIS7: why has all POST data gone missing? and this hotfix by Microsoft

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