Hi Michael,
I've personally been running a nanoBTS (1900 band) with latest master of all standard components a few days ago, and everything seems fine.
Furthermore, osmo-gsm-tester is running nowadays tests against 2 nanoBTS (1 in 1900 band, one in 900 band) successfully with latest osmocom master in https://jenkins.osmocom.org/jenkins/view/osmo-gsm-tester/job/osmo-gsm-tester....
Of course it could be that your network configuration is not correct, or you may want to look at the possibility to change/update the firmware in your nanoBTS.
Check also with wirehsark the communication between the nanoBTS and osmo-bsc, you may be able to find some error reported in there which may give you some information.
Cheers,