We also had this one..
On our helpdesk we work with specialists. Each specialist has additional rights on his field of expertise. The BB helpdesk specialist had been given rights on the BB SQL db in the past.
Later on, all helpdesk people were given rights to the db.
They got these rights by adding the group in the sql db.
The specialist had higher rights on this db than the rest of the helpdesk.
Because he was added twice (via the group and via his specific user account) and with different acceslevels, he got this error when trying to logon.
So for me it was related to confilcting acces rights on the SQL DB.
Hope this is clear and helps