Existing shared mailbox ... does not exist

Hello,

Dovecot 2.2.26.0 Thunderbird 45.4.0 ESR

Scenario: We have a shared mailbox with e-mails in its subdirectories. There are no e-mails in the shared mailbox itself.

Example: /Shared <----------------------------------- empty /Shared/call_for_papers <-------------- empty /Shared/call_for_papers/AAMAS <-- populated ...more of the same...

Problem: When selecting /Shared, Thunderbird chirps this message:

"The current operation on 'Shared' did not succeed. The mail server for account @example.com responded: Mailbox doesn't exist: Shared/Shared (0.000 + 0.000 secs)."