Last night while playing I saw these big black planes in Cyrodiil - near Alessia bridge and outside 2 keeps. I'm including screenshots of them and map locations where I found them. I never saw these before so I thought it might give the developers working on FPS issues another perspective.
The planes were not always visible or visible from every angle, they just showed up and disappeared after I turned or got closer to them. I had a low FPS session but which was still playable (ca. 15 FPS) so I didn't re-log for very long time before first seeing these. This was on a PC client version 1.2.4 - so before the latest patch and the first (and only) time I ever saw this.
I didn't put this in a bug report since the game is down for maintenance, however, just as before, feel free to e-mail me with any questions.