Despite having a lot of space in my Proton Drive, some synced folder fails to show up on the recipients side. One detail is suspicios, namely, that all other folders have that small green checkmark symbol in the status column, and also work, whereas the failed one shows a small cloud symbol. Now i'm not sure, if Opus provides those symbols, or Proton Drive, and why they are different in this case. Re-uploading the folder several times didn't change the appearance so far. All items have been uploaded in the same way, by dragging them over to the sync folder in Opus.
Have you checked with File Explorer? Do the symbols differ?
Maybe the recipients don't have enough free space in their drives.
It looks the same in Explorer. And there is plenty of space on the other machine. 