Thanks I didn't know about that!
For future reference:
You can use "rclone about " to find out the free space left on a remote. I switched from FTP to SFTP and it works now, after some debugging. For SFTP rclone apparently uses the df -k command to get the free space.
If using a Synology NAS, SSH should be enabled for this to work. The shares should also be renamed to the name of the storage pool.