Folder "Folder_name" isn't making progress

Well, what I am seeing in the console window is the same exact message I see in webui:

23:57:58: Folder “Folder_name” isn’t making progress - check logs for possible root cause. Pausing puller for 1m0s.

But what does it mean? What the real issue here?

One of my drives started behaving flaky as of late, and the physical share in question is located on that drive. At some point during the last boot that drive/partition just became invisible in Win, like it does not exist. After I rebooted, everything seems to work, except now I see this message from syncthing.

What should I do in this situation?