I think you updated during a very short window where there was a bug in the updater. Sorry about that. I had the update live for about 30 minutes yesterday afternoon before discovering the bug. It was updating fine on J! 3.7 and 3.8, but not on 3.8.1.
Try updating to 3.8.1 again, everything should now be copacetic.
For future reference, whenever there is a point release of J! (like 3.7.x to 3.8) you MUST update Fabrik if you update J!, otherwise you'll have that problem with any "repeated" params on the backend, like multiple plugins. J! doesn't support being able to add multiple instances like that, so we have to override one of the built in classes. We've tried submitting PR's on their code to get our little change in there, but ... meh, you don't want to know.
Usually we have a Fabrik update ready to go the day J! updates, but this time round we got caught out (they released ahead of schedule, just after my 2 week trip to the UK), and there was a lot of changes - 3.8 is a "get ready for 4.0" release that changed a lot of internals in J! that we had to code round.
As of this release (3.8), we'll actually warn you if you don't have the right class override for the version of J! you are running, and tell you not to try editing anything in the Fabrik backend until you've updated.
-- hugh