As far as I can tell, that error box pops up at random times - anything from posting a picture to accepting a warmark in review.
A few times I've had it pop up repeatedly when trying to submit a waymark. I don't know if there is some value that is in the incorrect format, an invalid date for instance, or what. You might just look in your unfinished waymarks - it might actually be there and you can edit and finish it off.