Just sharing my experience now in adding a new device to my syncthing nodes.
Lots of web searches brought me to closed topics on this forum and other places, but no joy. I kept getting the message no matter what I changed, restarted setup of the new device, etc.
It turns out that my browser (Firefox) was inserting the GUI password into the password field of the sharing tab. So, I just had to clear that autofill from that tab when accepting the folder after adding the device, and problem solved.
HTH someone who has the same trouble.