Hi Andrew,
what I see in your log is that your osmo-bsc receives an Assignment Failure
message from the BTS:
lchan(0-0-0-CCCH_SDCCH4-1)[0x18b8650]{ESTABLISHED}: (type=SDCCH) Rx ASSIGNMENT FAILURE
So the reason for the assignment failure is closer to the radio. You may want
to look at the BTS logs?
~N