Existing shared mailbox ... does not exist

[Ruga]

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)."