I am not sure what I have to look for and I didn't change the initial config files (except for the 666 mode for kamailio_rpc.sock).
As far as Kamailio is concerned the socket files files do exist:
root@deb11srv1-kamailio:/var/www/siremis-5.3.x# ls -l /run/kamailio/ insgesamt 4 srw------- 1 kamailio kamailio 0 7. Jun 11:37 kamailio_ctl -rw-r--r-- 1 kamailio kamailio 5 7. Jun 11:37 kamailio.pid prw-rw---- 1 kamailio kamailio 0 7. Jun 11:37 kamailio_rpc.fifo srw-rw-rw- 1 kamailio kamailio 0 7. Jun 11:37 kamailio_rpc.sock
When I look at siremis/log/ERR.log I repeatedly get:
'06/07/2022','09:43:29','ERR','ErrorHandler','socket_bind(): unable to bind address [2]: No such file or directory',''
This disappears when I put the local file socket in /tmp:
<UnixSockLocal name="unixsocklocal" address="/tmp/siremis_rpc.sock" timeout="3.0"/>
which was probably done this way in previous versions of Siremis (before systemd private tmp config). In this case I get no error messages, but still no answers (as expected). The next step seems to get UnixSockLocal right.