I use 2 different phpbb forums. One I have updated from 3.0.9 to 3.0.10, the other is still on 3.0.9. The one that I updated has a problem with the memberlist.php display. If you go to the first page of the converted forum, for the memberlist page:
Click on one of the letters at the top (A B C D) to sort it by alphabetical username, it goes to a 404 error page. Oddly enough, if I click on one of the sortable columns in the memberlist (ie. posts column header), and open up a new sorted page, then click on one of the letters (A B C D), everything works fine.
The other forum memberlist, still on 3.0.9 version, works find even from the entry page:
I believe the only difference between the coding is the 'first_char' routines.