Re: 404 on everything except home. sweet home.
It sounds like a permalink issue. @malik, you said that when you switched your WP permalinks to something other than the default, WP stopped working. It’s probably the case that your provider is missing mod_rewrite or otherwise does not have the correct server configuration to support WP permalinks (which are, in turn, required for BP). Here is the page on permalinks from the WP Codex; there are technical details in the second part of the page: https://codex.wordpress.org/Using_Permalinks