Connection issue

For some reason I can't connect to my PG 10.6.1 Server (running on Ubuntu) from my Windows installation (run via Parallels on my Mac), no matter what app I use (tried psql, PGAdmin and my own app).

When trying to connect from PGAdmin 4 the server logs say:

2019-02-05 10:34:30.558 CET [31051] [unbekannt](at)[unbekannt] LOG: 00000: Verbindung empfangen: Host=ip5b4054ce.dynamic.kabel-deutschland.de Port=50262
2019-02-05 10:34:30.558 CET [31051] [unbekannt](at)[unbekannt] ORT: BackendInitialize, postmaster.c:4249
2019-02-05 10:34:30.648 CET [31051] [unbekannt](at)[unbekannt] LOG: 08006: konnte Daten vom Client nicht empfangen: Connection reset by peer
2019-02-05 10:34:30.648 CET [31051] [unbekannt](at)[unbekannt] ORT: pq_recvbuf, pqcomm.c:978

("konnte Daten vom Client nicht empfangen" means "couldn't receive data from client")
PGAdmin says the usual " Unable to connect to server:..."
No issue connecting from the Mac.
I see in the logfile that other windows users did connect successfully from their windows boxes so I am ready to believe that maybe my Parallels somehow disconnects the connection quickly or something.
Another strange thing is that if I try to connect from my Xojo app (which uses libpq internally) it says:SSL error: unknown protocol. I had no issue connecting last week from that installation so I am at my wits end...

Tia,

Max