It's important that you have a "default" profile for each of your web users. Most of the time the "Manage Web Users" screen ensures this. But not always, and it's possible that a Web Users data pump has this wrong.
To immediately fix the problem, you can run this SQL against your database in SQL Server Mgmt Studio:
UPDATE WebUserProfile SET ProfileCode = ""
WHERE (ProfileCode <> "")
AND (WebUserProfileTypeID = 1)
AND (WebUserID IN (select WebUserID from WebUserProfile where WebUserProfileTypeID = 1 GROUP BY WebUserID HAVING COUNT(*) = 1))