are you able to record an A interface trace ?
there, you would see which message fail, or where exactly the drop occurs.
from what i understand, you can’t see any detail yet, and the sdcch drop counter is not “pegged” to the ciphering phase exactly. So you are actually quite blind about what’s happening. The next steps for you are to
– do a A-interface trace.
– find the exact event-trigger for this counter