[Evolution] POP account does not show up anymore

[Chide <cg xs4all nl>]

Version Evolution: 3.28.5-0ubuntu0.18.04.1 

I created a new POP account. It worked.

Then I switched it off (Edit -> Accounts -> Mail Accounts). It
disappeared from the Mail overview.

Then I swtiched it on again (again with Edit -> Mail Accounts).
Unfortunately, the account does not appear in the overview anymore.
Hence, I cannot access my email. The problem persists, even after
restarting my machine.

Other accounts (a few IMAP accounts) work perfectly. When I switch them
on or off, they appear and disappear accordingly, as I expect.

How to solve this problem? Is it a bug?

Thank you so much.