Dear German people using Telekom, ive went through some data from guild members as they have this issue to, and it all comes down to the following below.
Traffic from Telekom customers to OctoWoW's server transits through GTT Communications at the Telekom↔GTT interconnect in Frankfurt. During evening peak hours, this link (or GTT's onward path toward FlokiNET/Iceland) appears to become overcrowded, causing packet loss and latency for Telekom-originated traffic specifically. Traffic routed via VPN avoids this link and reaches the server without issue.
This is a known category of problem Deutsche Telekom has a documented history of peering/interconnect capacity disputes with several major transit providers, where DTAG's outbound capacity to a given transit network is under-provisioned relative to demand, producing exactly this signature: ISP-specific, time-of-day-specific congestion that is invisible to other ISPs' customers.
So basically its because Telekom uses a specific routing provider, which doesnt have enough to deal with the load in the evening. (Telekom is just a cheap ass trying to save money not wanting to invest it seems)
The server is being moved to Amsterdam as Archer said a while ago.

<GM> [Archer]: We will be moving to Amsterdam soon so a lot of the people with ISP routing issues should be resolved
This should fix all your problems as it will be using a different routing.