Hi,
Combining multiple search and filter plugins together is generally not a good idea, and there is a very high chance they will not work together at all.
I assume the error message you are reffering to is this:
Headers already sent. You cannot change the session module's ini settings at this time in Unknown on line 0"
It sounds like, something, somewhere is trying to change the session settings via an ini_set(...) function call. Unfortunately there is no indication where this is triggered, as there is only printed in Unknown. Our plugin however does not use that PHP function whatsoever, so the issue is very likely triggered within something else.
I guess that while using the GET method, some query arguments are recognized by another plugin/code, and it tries to execute, but there is an issue somewhere.
I recommend searching for a function called ini_set within other plugin source codes – as it is the most likely cause of that error message. That will get you closer to the problematic code.