This is a quite interesting issue. Perhaps it may help if we start by trying to understand what caused the issue and then we will consider how you might recover from it.
One of the things that Cisco wanted to do as they released version 15.0 was to improve the security of passwords in IOS. So they designed and implemented a new "type 4" password which they intended to replace the previous "type 5" password used for enable secret or for username secret passwords. So in those versions of 15.0 if you configure enable secret or username secret for passwords the IOS would create the new type 4 password.
Then you reacted to some problem and loaded an older version of IOS. 12.2(55) does not understand the type 4 passwords. So as your switch booted on this version of code and it read lines in the startup config that had type 4 passwords it did not understand them and marked them as invalid input. So the result is that your switch does not have an enable secret and perhaps usernames do not have passwords.
I do not know if you will be able to recover without doing password recovery but I have a suggestion which you can try. See if you can login on the console. It may depend on how your switch was configured whether it will let you do that and whether it will let you get to enable mode on the console. The default behavior was to allow the console to get to enable mode without a password. If your configuration includes aaa authentication enable default enable then the console may not work and in that case I believe that your only alternative will be password recovery.
HTH
Rick
HTH
Rick
