Are there rewrite rules in .htaccess of the root folder?
Random error is hard to debug, and we need at least some debug log for request results in 404, with rewrite log enabled, in order to look it further. It might have something to do with htaccess cache updating.
Is that possible to generate some debug log for analysis?
One possible fix is to create a static context for the forum, and move the rewrite rule from .htaccess to the context level configuration, without changing the rewrite rules. But, if the random 404 is cause by a bug in htaccess handle, the bug will remain there.