This may have already been mentioned, but during a recent upgrade to 18.2, I found that I could log into groupwise with the old client, but once the new client was installed, I was unable to log in. The same thing happened with the web access. Also, I upgraded the mobility server first, no problems, but once the groupwise install took place, mobility server was unable to login. After some thought, couple of whiskeys etc, I realised that it was necessary to upgrade the databases (post and domain) which has happened in the past.
Once I carried this out, no problems, so don't forget to upgrade the databases after the upgrade. Additionally, I could not find this mentioned anywhere in the documentation (though I may have missed it)_.