Problems connecting NC Windows 10 client to NC Server

Hi Community,

i just installed a NC server 20.0.1 which is running fine.
I also installed on a windows 10 system (2004) the most recent client (3.0.3).
For the server i have a signed certificate which is accepted by the following browsers: Chrome, FF, Edge, IE.
After starting the client a popup appears frequently:
Cannot connect securely to xxxxxxxxxxxxx:
I have to check to accept the certificate, then sometimes it’s ok for a while, sometimes the popup appears again.
This is what the error log from apache says:


[Wed Nov 11 12:45:43.564858 2020] [ssl:info] [pid 5413] (104)Connection reset by peer: [client 146.107.158.177:56133] AH01991: SSL input filter read failed. <===============
[Wed Nov 11 12:45:43.564858 2020] [ssl:info] [pid 5398] (104)Connection reset by peer: [client 146.107.158.177:56134] AH01991: SSL input filter read failed. <===============
[Wed Nov 11 12:45:57.759946 2020] [ssl:info] [pid 5402] [client 146.107.158.177:56138] AH01964: Connection to child 8 established (server xxx:443)
[Wed Nov 11 12:46:03.330704 2020] [reqtimeout:info] [pid 5169] [client 146.107.158.177:56136] AH01382: Request header read timeout <============

Any idea ?

Bernd