AttributeError: REQUEST

We recently migrated our Plone sites from Plone 5.2.14 to Plone 6.0.14. Since then we have been experiencing problems with our working copies. You can create and edit a working copy using the Actions button.
However, this working copy can no longer be discarded and the "Replace original with working copy" button is completely missing.
When I try to commit the working copy, I get the following error message:

Traceback (innermost last):
Module ZPublisher.WSGIPublisher, line 181, in transaction_pubevents
Module ZPublisher.WSGIPublisher, line 390, in publish_module
Module ZPublisher.WSGIPublisher, line 284, in publish
Module ZPublisher.mapply, line 98, in mapply
Module ZPublisher.WSGIPublisher, line 68, in call_object
Module plone.app.iterate.browser.cancel, line 51, in call
Module OFS.Traversable, line 364, in restrictedTraverse
Module OFS.Traversable, line 321, in unrestrictedTraverse

  • traceback_info: (, '@@plone_context_state')
    AttributeError: REQUEST

Who can help us solve this problem?
Katrin

is a standard content type or a custom one?

Standard Page