Hi,
it has been sorted.
The problems were 2:
1. wrong filename for a certificate file... my bad

2. the order of rsyslog's directives in /etc/rsyslog.conf is also important.
--
$ModLoad immark.so # provides --MARK-- message capability
$ModLoad imuxsock.so # provides support for local system logging (e.g. via logger command)
$ModLoad imklog.so # provides kernel logging support (previously done by rklogd)
$ModLoad imudp.so # provides UDP syslog reception
$ModLoad imtcp.so # load module
# make gtls driver the default
$DefaultNetstreamDriver gtls
# certificate files
$DefaultNetstreamDriverCAFile /etc/certs/mainca.pem
$DefaultNetstreamDriverCertFile /etc/certs/logserver-cert.pem
$DefaultNetstreamDriverKeyFile /etc/certs/logserver-key.pem
# UDP Syslog Server:
$UDPServerRun 514 # start a UDP syslog server at standard port 514
$InputTCPServerStreamDriverAuthMode x509/name
$InputTCPServerStreamDriverPermittedPeer *.domain.tld
$InputTCPServerStreamDriverMode 1 # run driver in TLS-only mode
$InputTCPServerRun 514
--
My 2 cents...
Martin