My Server and Problems

Hi, I have a huge problem with my server. When my friend tried to download mods, it permanently locked me out, and I can join back because of username verification. I tried turning off whitelist in an attempt to make the server public, but that didn't work either. So when I tried the whitelist, the OP, and the console, I was told the user does not exist for two accounts, and then they disappear from the whitelist. The player ID is correct, so I am not sure why it is acting like this.

Failed to connect to the server

Failed to verify username!

This has happened multiple times now.

This has also happened separately even without the mods, on Vanilla.