This is a known problem with a fix already pending for the next package update. It just hasn’t been published yet. The simplest solution to quickly fix the problem of not starting is shared in another thread:
Note there is some advice on other possible solutions in there as well. But all you really need is the one command from the linked post, entered through an SSH session as admin.
Tracking the issue happens at Syncthing Not Starting, after update to 1.19.0.25 · Issue #5142 · SynoCommunity/spksrc · GitHub.