Hi all, 
@nevj :
As for the logs, I indeed found them. barrier´s GUI provided them, but they weren´t of much use:
Failed to connect to server: Connection refused
This is what the client had to say.
But I got I running anyway (see: below).
I had ufw on my Lubuntu systems as well. Linux Lite favours firewalld and therefore I had to do some tinkering with that one.
Thanks for the confirmation. So it´s no barrier issue.
@all:
Well, I got it going in the end 
.
I talked to ChatGPT for some further help but couldn´t learn anything new. But it was good to learn some basic firewalld -related commands anyway.
I first tried
sudo firewall-cmd --permanent --zone=public --add-port=24800/tcp
sudo firewall-cmd --reload
on the server, which seemed a logical step.
I assumed that would´ve been enough. But it turned out I also had to do the same for the zone nm-shared.
No idea whether adding port 24800 for nm-shared woud´ve been enough. Anyway, I let the settings as they are for the time being.
And now indeed it works. 
I had to modify the default settings only on the server side. The client seems o.k. as it is.
So for the server it now looks like this:
sudo firewall-cmd --zone=public --list-all
public (active)
target: default
icmp-block-inversion: no
interfaces: enx001e101f0000
sources:
services: dhcpv6-client ssh
ports: 24800/tcp
protocols:
forward: yes
masquerade: no
forward-ports:
source-ports:
icmp-blocks:
rich rules:
and
sudo firewall-cmd --zone=nm-shared --list-all
nm-shared (active)
target: ACCEPT
icmp-block-inversion: no
interfaces: wlxe4beed63ad6d
sources:
services: dhcp dns ssh
ports: 24800/tcp
protocols: icmp ipv6-icmp
forward: no
masquerade: no
forward-ports:
source-ports:
icmp-blocks:
rich rules:
rule priority="32767" reject
… and now barrier works with firewalld active on both systems.
Thanks a lot to all of you for your help. 
Many greetings 
Rosika