some extensions suddenly become unavailable

I recently upgraded multiple Trixbox servers to version 2.2 and Sangoma driver to ver. 2.3.4-9 . Now some of the extensions become unavailable after a few or several hours, and I have to reboot the server to restore them to normal operation. When the unavailable extensions are rung from a ring group, messages in the log indicate that the extensions are not forwarded and don't have DND set , but simply can not be reached. Sometimes they might start being available again by themselves and then later become unavailable again. Could this be a problem with the database not being updated properly from one version to the next. I tried resubmitting each extension in FreePBX, but that didn't help.