I already reportet that at
UBB.Central:
http://www.ubbcentral.com/boards/showflat.php/Cat/0/Number/38908/an/0/page/0#38908After updating to 6.5 there were no problems with the new post-tracking system. But yesterday I opened a board with over 1000 unread posts. After a while, after reading maybe 100+ posts, the read posts stay "unread". Even after jumping into other boards, no single post is marked read anymore. btw: This is in flat mode and has nothing to do (?) with the known bug in threaded mode.
So I checked my cookies and found my w3t_tempread cookie filled up with 2969 characters. I heard about a cookie length limit of 4k or so. I really hope my suspicion isn't right...
Did you encounter such problems after reading a lot of unread posts?