After converting from Windows to Linux (both Build 9014), I had a few users that failed to load. Clicking them in the Users list (Manage -> Domains -> example.com -> Accounts -> Users) showed the following message:

The problem in my case was that the directories for those users had the first letter as an uppercase letter, but SmarterMail was looking for all lowercase. Windows (usually) treats "Username" and "username" the same regardless of case so it didn't matter before, but Linux (usually) doesn't. I renamed those directories to all lowercase and reattached the users (Attach User on the same screen mentioned earlier) and they were fixed.

Just thought this was worth mentioning in case anyone else runs into it!