Two more things you can test:
1) Remove some users from UserIdTable and check if ioftpd still crashes. Try to find a single user which causes ioftpd to crash. Then take a close look at this user's file.
2) Overwrite all user files with the one from ioftpd download. Test if your ioftpd still crashes. If it does, the problem is not in /users directory. If it doesn't, overwrite only half of the users - until you can find which users exactly cause the crash.
HTH
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