It’s been awhile since I’ve seen a search related error but our search tracking egnine was updated recently. I confirmed search is working on my test theme and twentysixteen. It seems theme specific. Try turning off search tracking in Inbound-Pro->Settings->Leads and see if that restores the feature.
I am testing your plugin (WP Landing Pages) and am having exactly the same issue, but cannot find any basic leads settings. Are you referring to a different plugin above?
I run the Iconic One theme and have deactivated all the plugins, reactivated one by one, and isolated the search disabling to the Landing Pages plugin. Do you have any suggestions to what I can do next?
I’m pushing a new version of landing pages that should take care of this issue. But the quick-fix would be to install Leads plugin by Inbound Now and use it’s settings to disable the search tracking feature.
I have used the work around from Hudsom Atwell’s post to get the search working.
However, the root issue still exists when the tracking feature is enabled. I am running Inbound Now Pro Version 126.96.36.199.7 on WordPress version 4.8.1. Issue is prevalent on our development, staging, and production sites. Each site is basically identical at the moment.