"invalid mailbox format" with upgrade to 2.3.3

[Andreas Hasenack]

I just upgraded a home system from 2.2.12 to 2.3.3 and started getting the 
"invalid mailbox format" error. I then ran reconstruct  and it worked after 
that.

Before the reconstruct, if I downgraded back to 2.2.12, things would get back 
to normal.

I remember seeing a similar problem someone had with an upgrade to 2.3.x in 
this list but I can't find it in the archives (or I'm mistaken).