Manage subscription not working

I am facing a problem that already occured and remain unsolved on another site before:

Visitors can subsribe to a list without problems.

When I send a newsletter to the list subscribers can “Unsubscribe” without a problem.

But they cannot “manage their subscription”, or to be precise: they can unsubscribe on that form and submit it, but then they are taken to the start page and their edits to the list are not saved (they are still on the list).

(The same happens to subscribers that are regular users on that site: they can subscribe – without being logging in – but they cannot manage their subscription.

We are using the latest WP version and DIVI theme.

Did anyone have the same problem and knows a solution?

Thank you

Klaus