Steps to reproduce:
1) Get an ordinary user XYZ.
2) Go ACP -> PERMISSIONS tab -> FORUM BASED PERMISSIONS -> Forum moderators -> select forum -> Add users (XYZ) -> Add permissions -> grant Topic actions -> Can merge topic permission.
3) Try to merge topics under XYZ account from the topic's "Quick-mod tools" menu and wait for redirect.
4) Get the error:

General Error
Module not accessible