Update: One night after signing-in and out of my iCloud account, many files got synced to my computer from iCloud but nothing got uploaded. I checked the logs of the cloudd process with "log stream --process cloudd PID" but found nothing helpful. There is one message "cloudd: (CloudKit) CloudKit SQLite Pool Drain" which appears often and which captured my interest but I found nothing on google about it. There is a potential reason for iCloud being stuck in "waiting to upload". Some month ago, maybe around the time iCloud stopped uploading, I worked on a github repository which was included in a iCloud subfolder. My computer van was making lots of noise and then I realized iCloud was having problems syncing many changes, for example after every compilation. So I moved the repository out of the iCloud folder which makes sense since it is already backed up at github. Maybe iCloud never recoverd from that.

For me signing-in and out of iCloud on my macbook pro did not help. The result was that the iCloud Drive folder is now empty and everything is saved to iCloud Drive (Archive). If I add a file to iCloud Drive it is not uploaded but stuck in "Waiting to Upload". Maybe it is related to the macOS Big Sur update. I would try if resetting / reinstalling helps but then I would loose all changes since January which are not yet uploaded. I'm a bit surprised there was no warning that all my files were not synced for 3 months. Maybe apple is working on it.

No upload since January this year. When I take a picture on my mobile phone it gets synced to my macbook pro but since January no file was uploaded to iCloud from my macbook pro this year. According to System Preferences / Apple ID / iCloud Drive, there are still 66 GB storage available. I am unsure if I should disable and enable iCloud Drive since the following warning is displayed. "If you turn off iCloud Drive, all documents stored in iCloud will be removed from this Mac."