Having some strangfe behaviour that I can't explain.
In a certain topic, in a closed part of the forum, the admin can't reach the second page (messages >15). The link [2] directs to page 2 but ends up on page [1]. This is only for admin, only this topic and only in Firefox latest version.
I'm using mod-rewrite and the topic has had some messages removed. It used to be >15, showing page 2 and after deletion went to <15 still showing [2]. It now has again >15 messages but the link or the page do not work. A hard refersh on the browser cache doesnt solve the problem.

It is not really an issue, but I would like to understand how this can occur. It seems it is a local problem, affected by browser version and user. How is this possible? Is it cookie related?

If you can't understand how it occurs, nobody will. Sorry - regarding our forum rules, if you can't provide step-by-step instructions to repeat the problem, we can't solve it.

Actually it's not because of our rules - it's because of human logic.

But it's definitely not the cookie's problem. It's obviously something related to your mod_rewrite rules. Try to launch forum without them, without mod_rewrite at all - and see if you can repeat the same. If you can't - the problem hides there.