Happened all the time yesterdat playing on Series X. Whenever I try and take a screenshot, I get a popup saying Photo Share Failed, and error code 0x30900191. I can't find anything online, but it's especially frustrating because it only appears after you get out of photo mode (so you "lost" your shot entirely)
If it's useful info, I verified that I could correctly connect to Ubisoft Connect via the game menu without any issue.