You'll probably need to go through your banned_users.cfg and bans.txt file manually and fix the ones that were broken.
In banned_users.cfg, remove the "STEAM_ID_PENDING" line, if it exists.
Then, in the bans.txt file, just find any SteamIDs that look like: "STEAM_0:1:123456 (Player Name)" and shorten it down to be the SteamID only, then double check that the SteamID is in the banned_users.cfg as well.
Save both files, then immediately restart your server so the changes don't get overwritten (or move the files off your your server, edit them, then transfer them back on.)