This particular problem has been solved.
It was an internal problem. The virtual server default pool was deleted.
Our forwarding policy did a forward to virtual server (pool was set to spaces).
In our irule HTTP_REQUEST we felt in the default option (url was not defined in iRule - only in policy), where we did a test comparing the pool with the default pool, which was positive and resulted in the error message.
Seen this problem we restored the situation (and also the default pool setting) - all OK.
We also did a reorder-update of the policies because we had before problems when the "forwarded to" virtual server was not active, which resulted in a LB_FAILED condition. When the virtual seerver became again active, we still got the LB_FAILED condition. To resolve this problem we had to do a reorder-update of the policy rules.