This indicates an attack attempt against an Authentication Bypass vulnerability in SolarWinds Serv-U FTP software.
The vulnerability is due to insufficient sanitizing of user supplied inputs. A remote attacker may be able to exploit this to create arbitrary users with elevated privileges (administrator) on the server.
All Serv-U versions prior to 15.2.3 HF2
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.