I start syncthing from the command line. I didnt try systemd yet. I installed the package through snap.
The problem occurs here:
[ACPUR] 19:25:39 INFO: Error while trying to start filesystem watcher for folder mlvm5-u56w2, trying again in 1min: open /media/slvst/D2TB/allo: permission denied
See below for the full log: https://pastebin.com/SwVN06zh
Also, I tried to run syncthing with sudo, and then it works! Then it can process the file on the external disk (/root/media/slvst/D2TB/allo/)