All extensions lost registration due to res_pjsip.c: Error 120032 'Broken pipe'

Overnight, all extensions lost registration with the following error:

ERROR[8039] res_pjsip.c: Error 120032 ‘Broken pipe’ sending OPTIONS request to endpoint XXXX

Asterisk 20.5.0
FreePBX 16.0.40.7
Endpoints registered PJSIP w/ TLS
Trunks are PJSIP UDP

What could have triggered this?

I also experienced the same issue on Asterisk 18.19 twice already.

Asterisk just stops responding to packets then.

In the /var/log/messages, I do see this:
kernel: TCP: request_sock_TCP: Possible SYN flooding on port XXXX. Sending cookies. Check SNMP counters.

(My clients are registering using TCP, instead of UDP.)

Does anyone has a clue what could cause this?

A denial of service attack

It seems not… Our machine is firewalled and the traffic is coming from a trusted source.

In sysctl.conf, I already set the following values, but still having the same issue:
net.ipv4.tcp_max_syn_backlog = 4096
net.core.somaxconn = 4096

I think its a different error but do you have max files high enough as well?