Error : 8629 The query processor could not produce a query plan from the optimizer because a query cannot update a text, ntext, or image column and a clustering key at the same time.
This error happens usually when multiple rows are updated and their clustering key is part of update. If multiple row of which clustering keys are updated there is always possibility that they end up having same value, which will violate the clustering key requirements.
Update one row at the time in this situation. Use while loop or cursor (I do not recommend cursor usage)
Reference : Pinal Dave (http://blog.SQLAuthority.com)