It’s still not entirely clear what your existing setup is like to make https://xxxxx.fr/syncthing work as intended with HTTPS enabled, but to re-disable HTTPS for Syncthing’s GUI…
After shutting down Syncthing, edit your config file. On an RPi, it’s most likely ~/.config/syncthing/config.xml or ~/.local/state/syncthing/config.xml if it’s been left to the default location.
In config.xml there’s a line that looks similar to the following:
<gui enabled="true" tls="true" debugging="false" sendBasicAuthPrompt="false">
Change the portion tls="true" to `tls=“false”'.
Alternatively, use Syncthing’s CLI interface to reconfigure the running instance, e.g. to disable HTTPS mode for the GUI:
syncthing cli config gui raw-use-tls set false