I would be looking at what changed recently did windows do a system update. If there is more than one computer try the others to see if the issues happens on them. On a phone if the same issues happens and on all computers then it’s a ISP provider issue or router setting.
good news from my side: now everything runs in every browser!!!
Ultimately, the problem was a configuration in my router, where the function ‘IPv6 support’ was activated. it has never caused any problems. However, there seem to be communication problems on the way from me to the server, but I can’t tell whether it’s with my router, my provider, or the VH server.
after deactivating this function, the current chrome versions now also query the connection to the VH server as ipv4, and now everything works here.
thanks to @kaya for the support and @jabbath1987 for the help with the search. there were a few puzzles to be solved and configurations to be compared until we were finally able to get closer to the cause
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Well the problem is that nearly all ISPs have some flaws in their IPV6 support. And Chrome priorizes IPV6 over IPV4 in the newer versions. So for now it is recommended to deactivate IPV6 in router so the fallback of IPV4 will work.
IPV6 vs. IPV4 always causes me headaches. It is not easy to understand and even harder to handle when you have both in your WAN or LAN
We at VHTV are pro-IPv6, supporting & enf_____g it both publicly & internally. But we do provide both v4 & v6 for everything. And yeah, it’s pain in the ass to maintain, and, unfortunately not everyone can do it properly. It’s a pity such organizations as ISPs fail the IPv6 support so often, though they are those who should do it properly & show example to everyone else within their networks.
My advice is to contact your ISP anyway as it may help resolving such issues for others in your ISP network. Please do not hesitate to contact us in case your ISP reports it’s something to do on our side - we’ll be happy to investigate and improve our services in case it’s needed.
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It would be paradise if every service and internet provider would do it. Especially in Germany it is really developing country with IPV6.
Fully agree 
Haha you are funny. They will take three weeks to answer, then do not understand anything and all they will do is send a tech guy who will check the connection coming from ISP into the router and that is it.
You know if I have problems with my Internet it is me telling the hotline what is wrong 
(I can see it in my Router what channel of the coax cable is making the problems)
So I fully understand what you mean but with German ISPs impossible 