PHP fatal error on assigning reviewer after 3.1.2-4 update

I just updated my site from version to 3.1.2-4. Everything went smooth until I try to assig a reviewer to a Submission. I try to search for a specific reviewer and I get the following message:

An unexpected error has occurred. Please reload the page and try again.

The error in the PHP log is this one:

[Thu Dec 12 11:08:22.898966 2019] [php7:error] [pid 24580] [client] PHP Fatal error: Cannot declare class UserHandler, because the name is already in use in /var/www/html/mlsjournals/api/v1/users/ on line 0, referer:

I’m using a custom template for the front-end and a custom plugin. Already tried disabling them both but to no avail.