Getting "Internal Server Error: Function 'dump' does not exist" when accessing/creating custom fields
Adding as an answer because it was confirmed via a support ticket that a plugin was at fault.

Twig's dump method is only available in Craft when devMode is enabled. 

Turn it off, reproduce the problem, then search your craft/storage/runtime/logs files for [error].

The stack trace listed in that error log will show which plugin is causing the issue.

Chances are that plugin is calling Twig's dump somewhere in a template, assuming it will always be available, when that is not the case.