Just ran into a problem with legacy site running on c5 v5.6.4.0 after the database server was upgraded to MySQL v8.x.

Because "Groups" is now a reserved word, a few files need to be touched:

concrete/core/models/groups.php
concrete/core/models/user.php
concrete/core/models/permission/keys/custom/access_user_search.php
concrete/core/models/search/group.php

Any occurence of "Groups" must be changed to "`Groups`"

Just adding this here in case someone is looking for a solution.
This issue is being taken care of here:
https://github.com/concrete5/concrete5-legacy/issues/1980...
A PR has already been accepted