But I think that the search function still doesn't work as it should
That's unrelated to the issue DdmrDdmr brought up.
Search has a number of quirks, and I'm not going to fix them all right now. In this case, your search didn't work as expected because words appearing in a large percentage of posts are not indexed within posts at all. Currently, there are 181 unindexed words, and "bitcoin" is one of them. So if you search for "bitcoin", it will not even look for posts containing "bitcoin" in their bodies, though it will search topic subjects, and it will
highlight "bitcoin" in any posts found via subject-search. As another example, if you search for "bitcoin candy", it will search for topics with
[both bitcoin AND candy in their subject] OR [candy in their body], even though it will
highlight both "bitcoin" and "candy" in message-bodies in the results. (Furthermore, this will exceed the result limit, so you will actually only get an arbitrary subset of the possible results.)