After upgrading to 1.9, Syncthing for Windows no more support the ‘\\?\UNC\’ path format.
Suppose to have a server ‘servername’ with a share ‘sharename’ with a subdirectory ‘folder\subfolder’ inside.
In Syncthing you would add a folder and specify the path ‘\\?\UNC\servername\sharename\folder\subfolder’.
Syncthing 1.8 is able to succesfully work inside this path and keep the files syncronized but as you upgrade to the 1.9 version you get the following error:
2020-09-11 11:25:23: Error on folder TestUncFolder: folder marker missing (this indicates potential data loss, search docs/forum to get information about how to proceed)
For references about UNC paths see: