Ignored filed and folders

I have some folders and single files, which are marked as “ignored” in the file explorer. So they are not synced, which is annoying. The “Edit Ignored Files” list is empty. When I start “Force sync now” I can see some error messages like "Server replied “423 Locked” to “PUT …server URL…” (Sorry, the client doesn’t show the whole message)

How to bring these items back to beeing synced?