Not working using flatpak on rasp pi 4B

[BramEPC]

At startup, after the initializing message he reports something goes wrong and I should check "the logs" but nothing appears on the command line even if I start with
flatpak run --command=warpinator org.x.Warpinator --debug

I does happen during or right after the Auth: Starting local cert server -- auth.py step, so it seems there is some error reporting missing there.
I there are other logs somewhere , best add the location to the gui error message so users know where to look, and/or to the github description.

If I start other warpinator clients on the network I do see them but I cannot connect with them (exclamation mark in red) .
At that point the cli does generate some SSL errors though:
2023-03-04 22:41:40,619::warpinator: Initializing Warpinator
2023-03-04 22:41:40,628::warpinator::DEBUG: UI: Creating window and status icon -- warpinator.py (line 987)
2023-03-04 22:41:41,043::warpinator::DEBUG: Starting network monitor -- util.py (line 405)
2023-03-04 22:41:41,044::warpinator: Starting server on 192.168.43.211
2023-03-04 22:41:41,045::warpinator::DEBUG: Server: starting server -- server.py (line 240)
2023-03-04 22:41:41,058::warpinator::DEBUG: Auth: Creating server credentials -- auth.py (line 230)
2023-03-04 22:41:42,929::warpinator::DEBUG: Auth: Starting local cert server -- auth.py (line 390)
2023-03-04 22:41:50,829::warpinator::DEBUG: UI: start discovery timer (no remotes) -- warpinator.py (line 742)
2023-03-04 22:41:54,285::warpinator::DEBUG: UI: Discovery timed out (no remotes) -- warpinator.py (line 758)
E0304 22:43:20.546869162 16 ssl_transport_security.cc:1439] Handshake failed with fatal error SSL_ERROR_SSL: error:10000416:SSL routines:OPENSSL_internal:SSLV3_ALERT_CERTIFICATE_UNKNOWN.
E0304 22:43:20.929740568 16 ssl_transport_security.cc:1439] Handshake failed with fatal error SSL_ERROR_SSL: error:10000416:SSL routines:OPENSSL_internal:SSLV3_ALERT_CERTIFICATE_UNKNOWN.
E0304 22:43:20.929881488 16 ssl_transport_security.cc:1439] Handshake failed with fatal error SSL_ERROR_SSL: error:10000416:SSL routines:OPENSSL_internal:SSLV3_ALERT_CERTIFICATE_UNKNOWN.
E0304 22:43:20.929961225 16 ssl_transport_security.cc:1439] Handshake failed with fatal error SSL_ERROR_SSL: error:10000416:SSL routines:OPENSSL_internal:SSLV3_ALERT_CERTIFICATE_UNKNOWN.
E0304 22:43:20.930039685 16 ssl_transport_security.cc:1439] Handshake failed with fatal error SSL_ERROR_SSL: error:10000416:SSL routines:OPENSSL_internal:SSLV3_ALERT_CERTIFICATE_UNKNOWN.
<...>

So probably some certificate problem but I don't get much luck googling this.
The clock on my pi is correct.

I tried installing gufw and adding the firewall rules but this did not change anything.
For now I disabled it again.