As I understand, this also affects all other forum user, not only me - right?

Ok, I will try to return to the previous version of the string tomorrow. Or maybe, there is no need to do that if I already solved the problem with my login? Wait until a forum user reports the problem with their login?

Yes of course this affects all users.

I changed the hashAlgorithmType back to SHA1 and was able to reset my password using the old security question. Now I know that I must not change this parameter in the future upgrades 😉

Is there any big benefit in using the new SHA256 hash algorithm? And is it bad that I did not set a personal machine key for our forum when I initially established it? As I understand, now I need to live with the current security settings as they are "till the very end" and never change them in the future.

SHA256 is more Secure then SHA1, but still you are safe to use the old hash algorithm.

Only when you switch to a different server it's a problem not to set a machine key