I am running Dropbox on a macOS and a Linux (Manjaro) computer. Recently, I noticed that certain files (namely images created with Photoshop CS6) do not sync onto the Linux computer. In the web interface and on the mac everything is fine. I am on the basic plan and still have around 25% of my storage space left, so this cannot be the issue. I also increased the number of watches files (fs.inotify.max_user_watches) to 100'000 which did not help.
It seems like I am not the only one with this issue. This thread and this one mention exactly the same problem but both of them do not provide any solution. I would be very happy if Dropbox could provide some support for this issue!
I finally figured out that Photoshop CS6 on macOS by default writes a resource fork to the files it saves. More recent versions of Photoshop do not have this behaviour. Apparently this resource fork is not supported by Dropbox (see here), hence the synchronisation issues.
Using the xattr command as suggested in this stackoverflow answer I was able to remove the attribute created by PS CS6 and the files started syncing as expected. Moreover, there is a setting in PS CS6 that prevents the creation of the resource fork when saving a file. This should avoid any syncing issues down the road.
I hope this helps other people with the same problem.