The only enumerable code I can see there is the Linq Count method calls:
count = dateNotificari.AsEnumerable().Count();
count_notificari_trimise = notificari_trimise.AsEnumerable().Count();
So what I would suspect is happening is that one or other of the two collections
dateNotificari
or
notificari_trimise
is being modified in a different thread during the Count operation. That would cause the "operation may not execute" error, but unless you are doing it pretty often I'd expect it to be an intermittent problem rather than a regular thing.
If you are modifying it outside the thread, then you need to look very carefully at your whole scheme - it sounds like it's not thread safe at all.