I found a solution in another topic but I can’t call it simple. When you install syncthing it has “autoupdate” option turned on already, and it immediately updates to the latest (broken) version and that’s it, you can’t run it anymore.
So right after installation (dont run syncthing yet) you have to change config.xml file and disable that autoupdate option. I had to connect to nas via SSH, download that file and upload a changed one.
If somebody knows an easier way to disable autoupdate (maybe via some synology scheduler command?) then please post this info here.