2 solutions apparently solve our issues.
- The database option "Is Read Commited Snapshot On"=True (By default, it's false)
- The database option "Allow Snapshot isolation"=True + read done using snapshot isolation level. We tried the read using snapshot isolation before, but did not know about this db option. I still do not understand why we don't get an error when reading with disabled isolation level?
More information on http://www.codinghorror.com/blog/2008/08/deadlocked.html or on
MSDN: http://msdn.microsoft.com/en-us/library/ms173763.aspx (search for READ_COMMITTED_SNAPSHOT)
http://msdn.microsoft.com/en-us/library/ms179599%28v=sql.105%29.aspx