Debugging dovecot acl shared mailboxes not showing in thunderbird

When you can't get ACL shared mailboxes visible with Dovecot and Thunderbird here are some debugging tipps:

Thunderbird fetches the ACLs on startup (and maybe at some other interval). So for testing restart Thunderbird on each change you make.
Ensure the shared mailboxes index can be written. You probably have it configured like
```
plugin {
  acl_shared_dict = file:/var/lib/dovecot/db/shared-mailboxes.db
}
```
Check if such a file was created and is populated with new entries when you add ACLs from the mail client. As long as entries do not appear here, nothing can work.
Enable debugging in the dovecot log or use the "debug" flag and check the ACLs for the user who should see a shared mailbox like this:
```
doveadm acl debug -u [email protected] shared/users/box
```
- Watch out for missing directories
- Watch out for permission issues
- Watch out for strangely created paths this could hint a misconfigured namespace prefix