Update from me: I also experiencing what PParth described. If I wait some time after getting the "Rejected"-message, I can upload pics again, but after I while it comes back.

 

For example: Uploaded a couple hundred pics via G+, got the "Rejected" and couldn't upload any pic, no matter format, size, album, whatever. Then waited 5-6 days and tried again. No problem, no rejection, even though I uploaded a couple hundred pics more! But now that wicked little red box is back again.

 

It indeed seems to be like a quota kind of thing. Interestingly though I can still upload videos without problem.

 

It's pretty pathetic that at Google no one seems to care about this. At least give us a "Cool it, we're working on it!". Or if there really is a quota, then make it public!!!