Extensive search of the internet (including this forum) provided no solution to my problem.
I have been syncing collections in LR Classic to the Adobe Cloud without any issues since many years. Currently I have >9,000 synced images, but suddenly LR doesn't sync my collections anymore. There is enough space in the cloud and everything appears to be ok according to Adobe (web and CC app).
Clicking the arrows to the left of the respective collections allows me to start or stop syncing, but the sync doesn't start. The top right panel in the catalogue tells me that all syncs are complete (although they are not). I can upload images to the Adobe Cloud via the web page but not from LR classic desktop. I have been trying to solve this issue for two weeks now, without success. What the hell could be wrong?
If there are any sync issues, click on them to know more, find the original for those images, and back them up safely before you empty the Sync issues from Lightroom Web. As long as there are sync issues on the Web, you can encounter trouble in syncing images & edits.