Searching the forums with a space in the search does not work. "%20" gets inserted so nothing comes up.

Searching for a keyword with an underscore also doesn't work. So nothing turns up when searching for my username, for example. I'm sure there are many more similar bugs. You can try using a search engine along with site:gog.com inurl:forum

Been like that for years. Honestly, as long as I remember. Not sure if it ever did work. Either way, don't hope for a fix, it's just another of the many, many broken things around here. Simply consider the search useless whenever you need to search for more than 1 word.

As others have said, been like that for years. Best bet is doing a site search with Google (or whatever search engine you use, assuming it offers that functionality)

I recommend you install Adalia Fundamentals and use its search instead:
https://www.gog.com/forum/general/adalia_fundamentals_fixing_gog_so_you_dont_have_to

I haven't used GOG's forum search in at least 6 years. It's been essentially broken since day one.