Same here. I had 4 pages of new posts. By the time I got to the bottom of the first page, I couldn't access the rest anymore.
From my observations of the board's behaviour, I've come up with the following description of how it handles "new posts":
The board tracks the date & time of the start of your current and previous "logical session". A logical session begins when you first login (after your previous session has terminated). A session ends when the board has not had a request from your browser for more than some period of time (the "timeout"). So, if you take too long reading or writing a post (for example, you get up for a cup of coffee or get a phone call), when you next try to access the board it is interpreted as the start of a new session and the date/time is reset.
"New posts" are determined by finding all posts that have been written since the start of your *previous session* (and before the start of your current session; otherwise, you'd miss messages). This is easy for the board as it only needs to store the date and time of your previous session. Keeping track of every individual message that every member has read would take a lot of resources. Also, the board knows where you left off no matter where you log in from; it wouldn't if it used cookies.
Assuming that my analysis is correct, I would recommend the following things to mitigate the negative aspects of this scheme (which I think might be relatively easy without requiring major changes to the board SW):
1. Increase the "timeout" value significanty. Users can reset it, if ever necesary, by explicitly logging out. Alternately, a button could be added to reset it.
2. Add a "search by date & time" option. i.e. find all messages posted after X. This way, if a user loses their place inadvertantly, they can at least easily find the messages they wanted to read.
3. When the board finds new messages based on the date & time of the start of your previous session, the date & time used in the query should be placed in the search results URL and subsequent requests using the search results should be evaluated using this saved query term, rather than the one stored in the user database. (I think this would prevent a search from resetting because of a new logical session.)
4. Consider scrapping the automatic session management altogether and replacing it with a system that requires the user to manually "bookmark" their place. If they forget to set their bookmark before logging off, they can still find their messages using #2 (above).
Just some thoughts...