I've noticed a strange bug on the forum using windows client. When I click on Next page or Previous page buttons often nothing happens and page doesn't change, but when I click twice on the same button (well, actually list item), page gets changed. It looks like it has something to do with loading or something.

Hello,
I suspect you aren't experiencing any bugs here, but just a limitation of the client forum.

Generally, on the client forum, you can't perform many actions quickly. You need to wait for some time to pass.

Translated into real examples, you can't for example upvote a post, then right away upvote the next post, or as you noticed, you can't go to the next page, then right away go to the next page again.

I suspect that was done to reduce the server load (in the past, the client forum had many bugs of being temporarily down or unavailable).

I would add some kind of "Please wait" message rather than silently failing, but there are probably good background reasons why it's the way it is.

If you need to navigate several pages forward (for example to find an older topic), I recommend using the Web forum on the site, it's more practical for that purpose, and also loads 50 topics per page instead of 20 as on the client.