I'm not sure when exactly it did happen, but for now, I'm listening to people only left-sided.

I did also find a thread where someone mentioned to activate 3D sound and move myself a bit around.

- 3D disabled, all centered - nada
- 3D enabled, moved myself around - nearly got myself centered, best what I got was a balance at 40%/60% compared to left/right

Additional infos:
- Saved my ID's
- Killed Teamspeak 3
- Deleted from FS (TS3 Folder)
- Installed again
- entered ID
- connected
- host sound OK 50%/50%
- Teamspeak 3 Testsound OK 50%/50%
- same issue 95%/5%
- Using Sennheiser 3.5 mm audio jack