Form components and submissions no longer visible after updating 3.2 (to 3.3 or 3.6)

After upgrading from webform 3.2 to 3.3 or 3.6 the form components and the submissions are no longer visible but still in the database. Reverting to 3.2 makes them appear again.
This happens to many sites which I am running. The problem can be reproduced only if there was a form created prior to upgrading >3.2 (e.g. 3.6). A new form created with 3.6 installed works correctly. It also affects other content types to which webform was assigned by /admin/settings/webform.
An inspection of the database shows nothing special. The entries in 'webform_component' do look the same no matter if the form was created with 3.2 or >3.2 (3.6).
It is a severe bug for productivity systems as they can not be updated without loosing all submission and form components data. It is not a problem for new sites starting with webform >3.3 (or 3.6).