I just tested it and it's changing just fine. Keep in mind that you should give it a full minute (or more) after making changes before closing Invision and restarting it to test. Otherwise, the settings may not be fully saved.
If you delete the login information, make sure that it changes to the default n/a nick with three •'s for the password. If it does, it should not keep trying to login or send login information as long as your nick isn't n/a. It should only send login information if your nick matches one of the nicks in the list.
Also, make sure you're trying to connect just to that one network when testing just in case you're getting that error from another network. It shouldn't be from another network, but just to be sure...