Why is registration not happening on asterisk?

Question

linux

Periodically, asterisk stops registering extensions. Restarting the asterisk/reboot server doesn't help. In the console (asterisk -rvvvvv) no registration attempts. In the dump on the PBX, you can see that register requests come from phones, but the PBX does not send anything back. Disabling the firewall does not help.
There are no errors in the asterisk logs, sip set debug on did not help either.
in /var/log/message nothing
in /var/log/secure nothing
Asterisk 11.16.0
FreePBX 12.0.76.6
OS: SHMZ release 6.5 (Final)
Ip address white
Where to look? Ideas?