Can't assign request

SDP 14105 updated, rebooted, but nothing seems can be solve this annoying problem.

Some tickets can't be assigned correctly. I can compile all informations of the ticket but when I select a technician to assign that request, SDP confirms the operation gone saving all the information but technician.

I can add a worklog and close the ticket, but the ticket remain without an assigned technician.

I don't know where I can find a log with other details about that, wich I can use to solve this anomaly.

In the System Log Viewer I can only see a "Sending notification failed for notifyType : EditQ" error, nothing else:

Sending notification failed for notifyType : EditQueueReqTechNotify for woID : 67839 Reason : {"response_status":{"status_code":4000,"messages":[{"status_code":4012,"field":"to","type":"failed","message":"Value not provided"}],"status":"failed"}}

Can anyone help me to solve the problem without using database query to force add a technician?

Thanks in advance.