iCloud not syncing any file (neither macOS nor iOS)

Hi,

my iCloud account isn't syncing any files, neither from macOS (10.11.6) nor iOS (9.3.3).

None of the programs are syncing, I've tried numbers, pages, iBooks - none are working.

I can manually upload files to iCloud drive, without any problems, then finder sees the file but doesn't load.

The same on iOS.

The screenshot shows the file uploaded to iCloud via Safari, the file in finder (not syncing, 'loading' forever) and my iCloud settings (sync with iWork etc. enabled).

Is there anything I can try?

Thanks for your support, br, Stephan