[Mac] Sync folder outside Nextcloud directory: Unable to sync documents in subfolders

Hello all,

I would like to sync my photos from the last year. My folder structure is:
2019/2019-01-02 event1
2019/2019-05-05 event2
2019/2019-07-20 event3
This folder structure is somewhere on my disk (not within nextcloud, because I only want to sync specific years).

In the Desktop client I added the folder 2019 on my disk to be synced with a folder also called 2019 on my nextcloud. The subfolders (event 1ā€¦3) are synced correctly, but the files in that folders are not synced, it just takes infinite time to do so:

If I copy the folder structure directly in the nextcloud folder, it works as expected.

Has anyone an idea what the issue could be?