I'm using the GOG Galaxy client. When I visit the Community forums (either of a specific game or the general ones) and want to search for answer to my question before posting it, I try clicking the "[magnifying glass] search" button at the top (and at the bottom), but nothing happens.
The "+ new topic" button works fine.

Am I the only one with this problem? What could be causing this?

It may be a better idea to post this in the Galaxy development sticky:

https://www.gog.com/forum/general/gog_galaxy_official_discussion_thread

That thread appears to be monitored.

I guess the bug you reported is a new one,
but everybody know the search function of GOG forum is broken for years.