Hi!
I tried to determine the cause with different configurations but no success yet. I would like to run a line-to-line debugging on the search files, can you provide temporary ftp access for that?
500 error usually happens when a php script stops with a fatal error at some point. In this case I’m suspecting that the template is either using a function with the exact name as the search plugin, or restricting access to certain ajax hooks or something similar.