Back it up, remove, see if it helps and if not, put it back. Easy as that, as everything's just files?
Better listen to the old saying: do not fix a working thing
. If Slack11 was working all right, why upgrade to 12, especially now that it's just come out and is still "hot"? Better stick to using 11 as long as it's sensible, which means long, or if you really insist on moving to 12, at least wait until most of the post-release bugs have been fixed. And before ramming on, breaking stuff like you apparently have, test
what happens if you can. An identical test-environment is always a nice thing to have.