This was caused by incorrect folder permissions for me. In my server error log I received errors such as:
Softexception ... wp-includes/js/tinymce" is writeable by group
Solution
Use FTP, CPanel, or some other means to go through ALL your WordPress folders and ALL subfolders and make sure permissions are set to 0755. Group and Other should not have write permissions.
Keep checking your error log until no more soft exceptions or any other file access issues are being received.
No idea if this will work for everyone, but hopefully a few people have the same cause as me. I compressed and uncompressed a WordPress install to migrate across servers without preserving permissions.
Best wishes
Paul