You don't need to read this page unless you got the GLIBC not found error.

Mod hashes passwords in order to store them securely in database. In order to do that, it uses a password hashing library. The recommended hashing algorithm is argon2 which requires libc6 (GLIBC) to be installed on OS. This is usually preinstalled on most systems.

If you own the server, try updating its software.

apt-get update
apt-get upgrade

If that doesn't help, try running

apt-get install libc6

That should take care of the problem.

For such cases, mod also provides a different hashing library (SimpleAuth 1.5.0 or higher).

Set the option useBCryptLibrary to true in SimpleAuth's experimental part of the config.

And that's it. If you have database files from previous server (which wasn't using the useBCryptLibrary option), be warned that no one will be able to login (since now you're using a different hashing method, which means that all passwords will be "wrong"). With other words, you cannot transfer database with Argon2 passwords to BCrypt passwords and vice-versa.