Error 304 (already logged in): my case

Godthab

My environment
iMac ( Late 2012 )
Graphics: Nvidia GeForce GTX 680MX with 2048MB GDDR5 RAM
Processor: 3.4GHz quad Intel Core i7
Memory: 32GB 1600MHz DDR3 DRAM SO-DIMM (8GB x 4)
ISP unchanged before/after the issue starts

Path toward the forced logoff and error 304
- Can happen anywhere in-world - PvE fields, PD, GD, Trials, Cyrodiil etc.
- Before the log-off I become unable to swap weapons; open doors; way shrine port etc. All other players around me stop moving. I myself can keep moving and do basic movements like jumping.
- Approx. 20 - 40 seconds after the above "pause" phase, I get logged-off and log-on screen shows up.
- I input my login password. The dialogue telling me "Error 304(6:3:1470:126) - This Account is already logged in." comes up.
- I input my login password again. This time I can proceed to character selection screen.

Effects caused by this unpredictable log-off issue
- Now I hesitate to participate in any group PvE activities, such as GD and Trials, since I don't wanna mess others' time up. This really s__ks.
- This makes my excitement in Cyrodiil a very intermittent one. Which also very much s__ks.

As far as I can remember this issue started haunting me since around 3-5 months ago. I hope it is not just me and ZoS is recognizing the issue.
If not, I will simply start to contact Customer Support.