chan_zap.c: No more room in scheduler

Asterisk with a Digium TE410P quad-span T1/E1/J1 with zaptel All works well most of the time except, every few days, the server stops communicating with the PRI. The log file shows the following error:

ERROR[23056] chan_zap.c: Asked to delete sched id -1???
ERROR[23056] chan_zap.c: No more room in scheduler

A google search takes us to a place that suggests this is caused by a version of libpri and that an upgrade to the latest version I obtained the latest version and compile it and … about 8 days later, the same errors show up and the box is cut off from the PRI again.

As a brute force workaround, I have a cron job that bounces zaptel in the dead of night, which seems to work while I try to get to the real root cause.

Anyone have any other ideas on what the real problem might be?