There's a bug in 5.1.0 in the Admin Access screen, which gives the error message "The value for column "ExtUserID" in table "Site" is DBNull".
You can work around this bug by clicking 'No' in the message-box that pops up and clicking 'Apply changes' again.
The bug is fixed in 5.1.1 - please email support at enterprisedt dot com if you want the prerelease. The official release should be within a few days.
