Yep, same here. I just tried updating to v2.9.2 and after the attempted update I’m getting this error message: “Briefly unavailable for scheduled maintenance. Check back in a minute.”
To clear this problem you need to delete the file .maintenance from the root directory of your WordPress install. If you’re on a Mac you’ll need something like Funter to see that hidden file. Unfortunately, I don’t know how to fix the fatal error you’re getting.
I just filed a support ticket with NF. I suggest you do the same: https://ninjaforms.com/contact/