Try a different device and report back? I can't reproduce the problem.
I've figured it out, the search "button" is actually 2 buttons.
I tend to click on the far right of buttons, and in this case that's a "Google" search, which yields the error.
If I click on the "Search" text, the left side of the button (and the larger part), it searches the forum.
This is true on all devices I have (Edge Chromium, Chrome on Windows, Safari on iPhone).
EDIT: The left side actually works intermittently, the right side never works.
The left side virtually always works on my phone, and rarely works on my Edge Chromium browser.
EDIT 2: The search term "KBC" never works, whereas the search term "Test" always works. I suspect there is some caching issue or something else similar going on as well.
EDIT 3:
The right button goes to:
The left button goes to:
Except if I search for "KBC" it always goes to the "google-search" link:
I replace the <query> in the second link to "KBC", it appears to work until you realize that it's still searching "Test" in the results (I imagine the 146776 represents a search and the q=KBC doesn't have any effect, possibly for cache purposes):
EDIT 4
So far it appears that:
1. Every search that yields no results, regardless of which button you click, goes to the "no results" page on the google search with that error.
2. Every search where you click on the right button regardless of results goes to the google search with that error.
3. Every search that yields results where you click the left button goes to a successful search page.