It's been disabled for now, because of the recent forum downtime. Basically theymos needs to regenerate the index. It's only temporary. He hasn't given a date to expect it to be functioning again.
Read about the recent downtime
here Thank you! Ok, let's wait.
Yes , @Welsh is right. This is the post of theymos regarding the forum search:
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Search is temporarily disabled because I need to regenerate the search index before it will be usable again.
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