Desktop clients not receiving new files

Hello All!

Hope someone can help:
I have a nextcloud 21.0.1 installed which connects to the files on an SMB share.
The desktop client does not receive any new files/pretends its all in sync even though files are missing. Rescan does not help to trigger the desktop clients. Files are visible on the web plattform.
Having a desktop client upload directly to the share will not trigger other clients to pull the fresh files or directories. Neither does placing the files directly on the SMB share trigger any pull.
Adding a file/directory locally triggers the desktop client to update this folder. Then it starts downloading. But only what is in that particular folder. I am guessing it is checking for conflicts and therefor gets a fresh directory listing and then discovers the not synchronised files.

The only way that the clients will download immediately is by uploading files directly via the web interface. Only then the desktop client initiates to pull.

The listener service is running. Rescan executed. Files are visible via web server. Force sync on client also has no effect.

Since this applies to Windows and Mac desktop clients of different versions alike and we did not change the clients, I am suspecting the error on the server side.

I hope that someone has an idea where to look to fix this issue.