This sounds like it’s deadlocking for some reason. Easiest way to debug it is to run it manually and capture the output, as I’m not sure how you’re starting it today.
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Stop Syncthing from running using your current method
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Open a new terminal, run
syncthing -no-restart. Leave it like that. -
When this happens again, open another terminal and run
pkill -QUIT syncthing. It will exit with lots of trace data.
Paste that data somewhere, here will do in a pinch. 