This is a bit meta, but when I search for more than a single word using the search function on this website or the advanced search function of the forum (restricted to the forum), I get an error message:
"Forbidden
You don't have permission to access /search/node/TERM1 TERM2 on this server."
Google doesn't seem to search the posts itself but only the forum overview.
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That's a known issue. See here for a workaround.
Ah, thank you, that makes sense.