Upgrading to Civi 5.11.0 in a Joomla 3.9.4 site (with templates & cache flushed) has left Civimail (via New Mail or Reuse) producing only blank pages (with menus and Civi's footer).
There is no error message with or without 'Enable Debugging' and 'Display Backtrace' turned on. However, just toggling 'Enable Debugging' ON makes the problem go away, while OFF brings it back, again with no error messages.
Any clues?