Some users get SSL error (only) on 5.15.2 (LTS)

[iiiCpu]

Some of my users on Windows get following error in requests for certain page

Error while reading: error:0609E09C:digital envelope routines:pkey_set_type:unsupported algorithm, error:0B09406F:x509 certificate routines:x509_pubkey_decode:unsupported algorithm

Strangely, this error falls into QNetworkReply::errorString with QNetworkReply::UnknownNetworkError code, not into QNetworkReply::sslErrors.
This error usually occurs while accessing GOST or other rare cryptography. But certificate for site states sha256RSA is used. At least, for Chrome and cURL.
OpenSSL binaries version does not affects the error. Tested 1.1.1f and 1.1.1i and 1.1.1m.
Also, this error occurs only in 5.15.2. Stepping down to 5.12.12 solves the issue. I'm currently moving to Qt 6 and it's no small task.

I highly doubt there's any simple solution there. So, I'm asking what extra data I might collect for a bug report? Assuming it's on users' PCs, and I'm very limited in tools.

[SGaist]

Hi,

A minimal compilable example would be nice for the bug report.

Did you also check with Qt 5.15.0 and 5.15.1 ? Just in case it something that changed with 5.15.2.