Can't edit modules after update to 3.6.3
From Joomla! Documentation
In some cases the backend module edit doesn't work or shows a white screen after upgrading Joomla.
Users see a white screen when editing a module in the backend (see: https://github.com/joomla/joomla-cms/issues/12456).
This pertains only to Joomla! version(s): 3.6.3
What is the cause
How to fix
Be sure to clear your browser's cache and your Joomla! cache.