EDIT 2020-02-09: THIS FIX HAS BEEN UPDATED. THE ORIGINAL FIX WORKED, BUT IT USED A POORLY CRAFTED SQL QUERY, WHICH WOULD OVERWHELM LARGE DATABASES WHEN SEARCHING WITHOUT A GROUP SELECTED. THE FOLLOWING CODE IS THE UPDATED FIX.
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For UBB.threads 7.7.3 only,

in admin/membersearch.php
around line 280,
FIND:
Code
		$extra .= " and t4.GROUP_ID IN ($g_inlist) ";
		$group = ",{$config['TABLE_PREFIX']}USER_GROUPS as t4";
		$groups = "and t1.USER_ID = t4.USER_ID";
REPLACE WITH:
Code
		$extra .= "AND t4.GROUP_ID IN ($g_inlist) AND t1.USER_ID = t4.USER_ID";


around line 320, AND around line 373,
FIND:
Code
	{$config['TABLE_PREFIX']}USER_DATA as t3
	$group
REPLACE BOTH WITH:
Code
	{$config['TABLE_PREFIX']}USER_DATA AS t3,
	{$config['TABLE_PREFIX']}USER_GROUPS AS t4


around line 325, AND around line 378,
FIND:
Code
	$groups
	$removedlist
	$nomods
	$extra
REPLACE WITH:
Code
AND	t1.USER_ID = t4.USER_ID
	$removedlist
	$nomods
	$extra