matrix-synapse: not starting & an error 500 in plinth (invalid config)

Created by: testbird

(This is on an apt-get install on clean armbian/buster. Plinth 0.20.0)

The matrix-synapse service did not start, and the plinth matrix page only showed a "500" error.

systemlog -xe contained:

ruamel.yaml.scanner.ScannerError: sequence entries are not allowed here in "/etc/matrix-synapse/homeserver.yaml" line71, column 12

and it read like this:

  - names - 
     client
     - webclient

instead of this working syntax:

  - names 
     - client
     - webclient

What could have caused this? I had not edited the file before.