Synology CloudSync fails: alternatives for bidirectional headless syncing?

Hi all,

I would like to synchronize data from my Synology DiskStation with my Nextcloud on the Internet. As it turns out, bidirectional synchronization with Synology Cloud Sync is not reliable: The initial synchronization seems to work, but after a few days I notice that changes from Synology Cloud Sync’s Nextcloud do not end up on my DiskStation. Sometimes the files end up on the DiskStation after hours. With one change, I gave up after a few days. From DiskStation to Nextcloud the sync seems to work.

With rclone I was able to transfer the broken states at least from Nextcloud to DiskStation. It also turns out that Synologys Cloud Sync, matches files that are considered changed after checking with rclone (after all, rclone supports md5 hashes with nextcloud!) and resyncs. To me, Synology’s software doesn’t seem particularly trustworthy.

Unfortunately, rclone is not a solution for me, as the program are only designed for unidirectional synchronization. But with my data there are changes on both sides.

Does anyone have an idea how I can keep my data (25,000 files) in sync?