Did some more testing with the settings.
First I have to retract my previous comment. I have Elite linked into Steam as an external game. With this only the launcher exe shows up then the game is not running, therefore my previous test was done with an odd resolution.
I followed the guide to downgrade to 1.7.15, I had an initialization error but everything was running fine.
1st: SteamVR version 1839100633 with odd resolution, small but visible distortion
2nd: SteamVR version 1839100633 with even resolution, no more distortion
3rd: SteamVR version 1576990468 (latest beta) with even resolution, no more distortion
4th: SteamVR version 1576990468 with odd resolution, small but visible distortion
All tests were done with ultra volumetric settings. SS-settings in SteamVR are 100% global and 156% (1908x1908) for ED. In ED SS and HMD are 1.
I'm not 100% sure if the 1839100633 version is 1.7.15 as Steam updated SteamVR after the first try and I re-copied the 1.7.15 folder.
Anyways, many thanks to Kyokushin for finding a solution to the problem.