I had to sync a couple of things between my PC and phone today and Syncthing just decided to stop working, giving me the “Error on folder/folder marker missing” error. Nothing changed and .stfolder is on my android. It’s not on my windows folder, and I can’t create one there due to an “Invalid file type” error from Windows 10.
Creating a file with only one dot at the beginning can be done in Windows Explorer by adding a dot at the end. The resulting file will not have that dot at the end. (Dont’ ask why that is ;)).
In current Version, it’s a folder, not a file (but I’m not sure if Syncthing really cares).