having same error when in plugins_install page if doing anything that needs a search… ( okay on Start page)..
Warning: http_build_query() expects at most 2 parameters, 3 given in /home/…./wp-includes/http.php on line 251
Everything was fine before ! the last plugin I activated was WASSUP so I deactivated but the problem still there. I did upgrade my php version from 4+ to 5.02 two days ago but I think (as far as I recall) I did not have this error afterwards!
Okay, I did another upgrade on my php version to 5.2 and all works fine now – the error disappeared on all plugin-install pges and I installed new plugin with no problem !