My Cloudstation app stopped working and would not synchronize files anymore. This happened both on Windows, and on Mac.
It seemed like the app was either permanently in a "connecting" state, or pausing itself.
It turns out that my Quick Connect became disabled at some point. I am not sure how or why; it had no account associated. I re-added that and enabled Quick Connect on my DiskStation, and Cloudstation started working again. This was especially odd because in the Cloudstation settings on my Diskstation, the status "Healthy" was displayed.