Remo

We are quite sure now that it must be a problem of the USG firewalls. We connected a client directly to the internet and the problems do not occur. 

Not really a solution but an update: The SSL VPN seems stable as long as there is no traffic from Site-to-Site. However, as soon as we are starting to access servers or services on the other site through S2S VPN, we are getting disconnected.

Thanks again @USG_User. I think we have no choice but putting a client in front of the firewall(s) and check if the instabilities occur 🙃

Thanks @USG_User, for your reply. The problem remains if the clients use SSL VPN and are connected to the internet through wires. If we use the native Windows Client and L2TP/IPsec, the connection is very stable. As mentioned previously, the S2S connection between our two main offices are stable as well. As if the this…

Thanks @Zyxel_Tobias for your quick reply. We manually removed the "connectivity check" through CLI which solved the problem. 

I think it's fixed already. Installing the latest Windows Updates worked for us.

Running version V4.38 on a USG40W this problem still exists. No warning or error appeared after password renewal. For us this is a reason to change the supplier.