Error with virtual index "Broken mailbox_count header"

Hi, we experience trouble with one of our virtual mailboxes.

dovecot-virtual:

* -INBOX/Trash -INBOX/Spam all

Error log:

/var/vmail/mailboxes/kirchhofer.net/tobias/virtual/all: Broken 
mailbox_count header```

This happens only with the filter form above. All other virtual 
mailboxes works fine.

Jakobus Schürz experienced something similar:

https://www.dovecot.org/list/dovecot/2018-January/110656.html

We do not have the panic message.

Is this probably something on our side or is this Dovecot?

namespace virtual { prefix = Virtual/ separator = / location = virtual:~/virtual }

-- collect@shift.agency