[marky]

Well, I’ve found the solution! Apparently, ModRewrite doesn’t count the query string as part of the URL, so it needs to be addressed separately. These rules did the trick:

RewriteEngine On

RewriteBase /forums/

RewriteCond %{QUERY_STRING} ^t[0-9]+)$

RewriteRule ^viewtopic.php$ /forums/topic/%1 [R,L]