[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 1…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?

Markus