Syncthing 1.7.1 on LG G6 with Android 9.
I don’t think that this is related with ST version 1.7, last successful synchronization was about in May. I’ve got several folders set up on this mobile phone, but only the “Camera” folder reports this problem.
The synchronization on “Camera” is set up as unidirectional (phone -> NAS). .stfolder exists both on the phone and on NAS. NAS does not report any error.