I get a WSOD when I use the search function on CiviCRM
I don't have any experience with CiviRules specifically, but the two things that usually clear up WSOD for me (on WordPress at any rate -- what CMS are you using?) are to clear out wp-content/uploads/civicrm/templates_c, or you can also try going to Administer -> System Settings -> Cleanup Caches and Update Paths, then click the Cleanup Caches button.

Depending on what version of everything you're on there's also the possibility that the CiviRules extension isn't compatible with your version of CiviCRM, although a quick glance at their releases seems to indicate they keep very on top of things.

It'd also be helpful to see the log details about the 500 error if trying the things above doesn't resolve the issue.

Hope that gives you some things to go on.