Creating new mailboxes

[Linda and Frank <franklin1 chello nl>]

I searched through the list archives and the FAQ but couldn't (or used the
wrong keywords) find the problem I discovered today.
My Balsa 1.1.1 can't open mail boxes I created. Whenever I create a new
mailbox (mbox) and try to move email to it, Balsa complains that it cannot
open the destination mailbox for writing.
However, mailboxes created by procmail it can open just fine.
I must be missing something obvious but cannot figure out what that may be.

Cheers,

Frank