There has been a temporary iCloud outage affecting some of our users. We have made several attempts to improve the issue, and while it has been resolved for some of our users, we are well aware that a lot of you are still experiencing it. We are working closely with Apple’s iCloud team right now to investigate the root cause and find a solution. This is our top priority right now and we will keep you posted.
Note that this iCloud issue is caused by iCloud servers repeatedly rejecting sync requests from the app, and is not related to the upgrading or subscribing to Goodnotes 6 in any way.
Feel free to also check out our general iCloud Troubleshooting guides for possible fixes.