Profile and membership synchronization problem
When the profile and membership synchronization is not up to date anymore the problem could be that the content databases where detached and marked as moved. When a contentdatabase has been detached the GUID changed and that causes the problem for synchronizing profiles.
To solve this problem you could use this stsadm command:
stsadm -o preparetomove -contentdb <DatabaseServerName>:<ContentDatabaseName> -site <URL from SiteCollection> -undo
After running this stsadm command, start a full crawl off the content databases.