Yeah, Solonari, I've gotten this issue a couple times before, but haven't been able to successfully reproduce it. I'll send the banlist to Megiddo and see what he thinks. What was the history with the ban STEAM_0:1:########? It appears twice in that table you sent me:
STEAM_0:1:########:
unban = 0
And then the messed up one:
= STEAM_0:1:########
Did you try unbanning it recently? Or was it a temporary ban that expired? Or was it just a regular ban that got messed up?
Before you do anything else, can you also send me your cfg/banned_users.cfg file, and your data/ULib/bans.txt file?
After sending me the files, you can go ahead and try to fix the problem by removing any instance of STEAM_0:1:######## in those two files, then restart your server. You can re-ban that user if you need to.
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(Also, be sure to check out XGUI
You can still view the banlist in case something like this happens)