Portupgrade and "Updating the portsdb"

[c0re]

Hello all!

I've got set of servers that uses NFS mounted /usr/ports. When I use
"portupgrade samba" on 1st server it says
[/usr/ports/INDEX-7.db: unexpected file type or format -- Invalid
argument] [Updating the portsdb <format:bdb_btree> in /usr/ports ... -
22601 port entries found  error] Remove and try again.
[Updating the portsdb <format:dbm_hash> in /usr/ports ... - 22601 port
entries found .........1000.........2000.........3000.........4000.........5000.........6000.........7000.........8000.........9000.........10000.........11000.........12000.........13000.........14000.........15000.........16000.........17000.........18000.........19000.........20000.........21000.........22000......
..... done]

Okay. It took 10-15 mins to rebuild.

Then I say "portupgrade samba" on 2nd server it says again
[/usr/ports/INDEX-7.db: unexpected file type or format -- Invalid
argument] [Updating the portsdb <format:bdb_btree> in /usr/ports ... -
22601 port entries found  error] Remove and try again.
and rebuild portsdb.

Why is it so?

Ports are updated via "portsnap fetch update".

/etc/portsnap.conf has
INDEX INDEX-5 DESCRIBE.5
INDEX INDEX-6 DESCRIBE.6
INDEX INDEX-7 DESCRIBE.7
INDEX INDEX-8 DESCRIBE.8

So while portupgrade rebuilds portsdb it's not possible use
portupgrade on 2nd server because later build process will fail on 1st
or second server.

What can I do with it? Why portupgrade always thinks that
[/usr/ports/INDEX-7.db: unexpected file type or format -- Invalid
argument]?

Thanks!!!