When I have unread posts (list of "new posts" on the top) and click on the button "mark all as read" a window without any text appears here on this board.
Check you error log for an undefined index error ... it is either missing a language string, or the language file was not loaded (1.1 ?)
It happened here, Spuds! :D
It's a previously reported bug.
http://www.elkarte.net/community/index.php?topic=4050.0
Then I'd say the site is not running the released B3 but a version thats a couple of PR's before it.
That could very well be.
/me is innocent!! O:-)
I'd update it but I'm still burning cell phone data :(
Galaxy 7? :D
LOL
I'll upload the files... soonish, maybe this evening or so.
LOL ... could be I have a Note 7 ... of course apple phones are not immune to producing a nice hot apple pie.
/me is fighting with too many decisions to take these days.