I ran across an instance of Asterisk the other day that was working fine except that none of the MeetMe conference bridges were valid. The conference bridges were previously working, but the server had recently lost power and rebooted. Whenever someone tried to use a conference bridge, the following message appeared on the Asterisk console:

WARNING[29733] app_meetme.c: Unable to open pseudo device

As you are probably aware, the Asterisk MeetMe application uses a Zaptel timer to keep all the voice channels in a bridge synchronized. This particular server doesn't have any Zaptel hardware, so it uses the ztdummy kernel module to provide the timing.

Come to find out, someone had installed a new kernel, but had not yet rebooted into that kernel. When the server restarted after the power outage, it used the new kernel which did not have the Zaptel kernel module installed. This was verifiable via the lsmod command. The solution was to rebuild and install the Zaptel module. After that, the module could be loaded without rebooting the server.