I know I already told you the solution that usually works for this but I just saw this bug report so I'll describe the solution encase other staff see.
This is a pretty common problem with groups. The solution is relatively simple-
Any staff just needs to make a group themselves and then do /adminjoingroup (should be the command, or similar enough) to force the bugged player into their group, and then either kick them using the group tab or /adminkickgroup - either will work, and then the player won't be bugged anymore.