There are some unquoted strings within the plugins services.yml file. Too many changes to list them all. Basically the fix is find all the strings that are (for example) %something% and insert single quotes so they appear (for example) '%something%'
This change makes the plugin compatible for phpBB =<3.2, which uses newer versions of Symphony.
Edit: As there are too many changes, even though slightly, I will be making the file available for download either separately or combined with a zipped release of my version (fork) of the extension. Not doing so just yet as there may be some other additions needed.