Are you using the latest version of UI.X (Both the add-on and style)? We added PHP 7.1 support a couple of versions back. If you are and still get errors go ahead and make a ticket at themehouse.com and we can take a look
Just upgraded to PHP 7.0.22 and everything seems to be working well on all my sites including my Xenforo forum. I'll have to wait until Ubuntu 18.04 LTS comes out to upgrade to a higher version of PHP as I only ever run the LTS server releases on my VPS. Hopefully by then any problems with recent versions of PHP will be fixed.
Thanks. So I'm my latest attempt to flip a switch from 5.6 to 7.0, I got the "an unexpected database error has occurred" message. Flip back, no error. If it matters, this was on a private board (password protected).