[Calgon]

https://sampwiki.blast.hk/wiki/Common_Problems#Server

Quote:

Players constantly getting "Unacceptable NickName" error but it is valid If you are sure you use an acceptable name and the server runs on Windows then try to change the compatibility option of the samp-server.exe to Windows 98 and it should be fixed after a server restart. Windows servers with high up time may also cause this issue to occur. This has been noticed of around 50 days of server up time. To resolve it, a reboot is required.

Confirm that your server isn't running on Windows and if it is, reboot the physical machine.