OpenSSL: decryption with private key _sometimes_ throws "error:02000079:rsa routines::oaep decoding error" - encryption

Question

We are trying to use openSSL to encrypt a string with a public key, and then decrypt it with the private key.
Curiously, the workflow sometimes works, and sometimes it does not work (for the same keys and texts) and throws an error.
First, the private and public key are generated by:
openssl req -newkey rsa:2048 -new -nodes -x509 -days 3650 -keyout key.pem -out cert.pem
openssl rsa -in key.pem -outform PEM -pubout -out public.pem
After that, the code below is executed.
#include <iostream>
using namespace std;
#include <cstring>
#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/ssl.h>
const int PADDING = RSA_PKCS1_OAEP_PADDING;
RSA *createRSAWithFilename(const char *filename, int pub)
{
FILE *fp = fopen(filename, "rb");
if (fp == NULL)
{
printf("Unable to open file %s \n", filename);
return NULL;
}
RSA *rsa = RSA_new();
if (pub)
{
rsa = PEM_read_RSA_PUBKEY(fp, &rsa, NULL, NULL);
}
else
{
rsa = PEM_read_RSAPrivateKey(fp, &rsa, NULL, NULL);
}
return rsa;
}
int public_encrypt(unsigned char *data, int data_len, RSA *rsa, unsigned char *encrypted)
{
int result = RSA_public_encrypt(data_len, data, encrypted, rsa, PADDING);
return result;
}
int private_decrypt(unsigned char *enc_data, int data_len, RSA *rsa, unsigned char *decrypted)
{
int result = RSA_private_decrypt(data_len, enc_data, decrypted, rsa, PADDING);
return result;
}
int main()
{
std::string textToBeEncoded="hi";
unsigned char plaintext[1024] = {0};
unsigned char encrypted[4098]= {0};
unsigned char decrypted[4098]={0};
strcpy((char *)plaintext,textToBeEncoded.c_str());
RSA *rsaPublic=createRSAWithFilename("public.pem", 1);
memset(encrypted,0,4098);
int en_result=public_encrypt(plaintext, strlen((char *)plaintext), rsaPublic, encrypted);
//fprintf(stderr, "Error: %s\n", ERR_error_string(ERR_get_error(), NULL));
RSA *rsaPrivate=createRSAWithFilename("key.pem", 0);
memset(decrypted,0,4098);
int de_result=private_decrypt(encrypted, strlen((char *)encrypted), rsaPrivate, decrypted);
//fprintf(stderr, "Error: %s\n", ERR_error_string(ERR_get_error(), NULL));
std::cout << "Result EN: " + std::to_string(en_result) << std::endl;
std::cout << "Result DE: " + std::to_string(de_result) << std::endl;
std::string decryptedString = std::string((const char *)decrypted);
std::cout << "DECRYPTED: " + decryptedString << " END."<< std::endl;
return 0;
}
Output:
At one time, the output looks okay and 0 (No error) is the only error code:
Error: error:00000000:lib(0)::reason(0) // error code=0 after encryption
Error: error:00000000:lib(0)::reason(0) // error code=0 after decryption
Result EN: 256
Result DE: 2
DECRYPTED: hi END.
At another time, the output is this:
Error: error:00000000:lib(0)::reason(0) // error code=0 after encryption
Error: error:02000079:rsa routines::oaep decoding error // ERROR after decryption
Result EN: 256
Result DE: -1
DECRYPTED: END.
Can someone help?
Best regards!