Steps taken:
  1. replace SET_HERE_FULL_FORUM_DIRECTORY_PATH with my forum path
  2. replace all [L] in my htaccess to [QSA]
  3. added both rules for htaccess
  4. added rule at the top of forums htaccess and replace ADDITIONAL_DIRECTORY to gallery which is my subfolder


I still encounter the same problem. In case I missed out previously, this is my testvb test site setup.