With PHP 7 and MySQLi support, UBB.threads gets bumped up to version 7.7.x.

TOC / Overview
UBB.threads 7.7.1 Changelog
Upgrading To The Current Version of UBB.threads 7
Styles Included with UBB.threads 7
What To Do Next? / Discussion

UBB.threads 7.7.1 Changelog
Version 7.7.1 includes several critical fixes when used with PHP 7.1 and newer. It is highly recommended that you upgrade from version 7.7.0 to 7.7.1.

UBB.threads 7.7.1 has been released and is now available to download in the Member Area.

Summary of Changes
[UPDATE] CP: Several entry fields in Primary and General configuration settings have been updated to only allow specific entry formats. ex, only number fields can accept numbers as entry. This should further help to prevent incorrect data types from being submitted.
[UPDATE] Updated Private Topics database Subject field from 60 to 255 characters.
[UPDATE] New Reply notifications now display the forum category name in which the reply was made to.
[UPDATE] Reformatted and optimized all CSS files.

[FIX] (PHP 7.1+) CP: Fixed error when in Member Editor when updating a member account and max threads/posts per page field is left empty.
[FIX] (PHP 7.1+) CP: Fixed error when manually approving new members.
[FIX] (PHP 7.1+) CP: Fixed error when exporting a list of email addresses to email.
[FIX] CP: Fixed setting the public posts and private message Subject length to a range outside of 10 to 150 characters in length.
[FIX] CP: Fixed customizing the posts and topics per page to a range outside of 5 to 400 characters in length per member in the Member Editor.
[FIX] Fixed creation of CREATE_NEWS permission columns during new installs of v7.7.0. See the "Special Upgrade Instructions" section below.
[FIX] Fixed email notification where sometimes the salutation would be missing the member name.
[FIX] (PHP 7.1+) Fixed new Member registration error when sending them an email.
[FIX] Fixed display of the Referrer icon (arrow) next to the Member name on the Whos Online page. It now correctly displays only when the referrer is different than your forum's domain.

2019-03-02 UPDATE:
ubbthreads-7-7-1p1.zip has been posted to the Member Area
This patch fixes:
1/ add the missing arguments in the email functions to prevent stack trace errors on PHP 7.1+.
2/ correct the "input=url" issue when attempting to submit changes in the Member Editor if the member has a homepage stored in their profile.
Download ubbthreads-7-7-1p1.zip and replace (overwrite) the files in your 7.7.1 forums with the files from within this archive.
As there are no new features added, your forum software version will not be changed or updated for this patch. It is intended as a drop-in replacement for the current files.
ubbthreads-7-7-1.zip has been updated within the Member Area
This is a release refresh and is intended to fix any previously reported install issues. If you're having problems with the ubbthreads-7-7-1.zip package you've downloaded prior to 2019-03-02, download this main .zip file again. This release refresh also includes within it the patch mentioned above.