Ubuntu Linux: No connection to TeamViewer Server - Please check your internet connection

Hi at all,

since a few weeks I have the problem, that I can't get a connection to Teamviewer-Server on several computer.

I get a message to check the Internet connection, but the Internet already works.
And by the why: I didn't use a proxy server.

I had this problem with last version 12.0.76279 and now with 12.0.85001, too.

I read in another topic about changing the DNS, but I don't know if I have done it correctly.
I changed these parameter in the network control under IPv4 and changed the DNS manually to 8.8.8.8
But the problem is still the same.

Any ideas, how I can check, what is the problem and how I can solve it?

Thanks a lot in advance.