Make sure the SSH login attempt is with a user account that belongs to admin account.
You can access the built-in web-based terminal available in Cockpit. Cockpit can be access without depending on Plinth. Cockpit is app that is enabled by default. It is available at https://<your_domain>/_cockpit/.
Please post the details of the 500 error page (although I believe the details are only available if you are a logged in user). That might give us further ideas.
See if disk is full, try clearing some space (say by removing some files from syncthing folders).