First of all – Please make sure that you have installed version 1.1.0 or newer of UABB. We have applied patch for this issue in our version 1.1.0. Please upgrade to it to run your website smoothly.
Meanwhile, to get back your website in working condition. Please deactivate the UABB plugin and remove it manually through your FTP program.
Login to the site via your FTP Program.
Go to the WordPress plugins directory and find bb-ultimate-addon folder.
Delete the bb-ultimate-addon folder from your server.