This was contributed by JLChafardet from the ev1forums:

How-to fix login problems in Plesk (Unable to open rpm db : [cannot open Packages index using db- ( - 30982 )]

This seems to be a common plesk issue when RPM gets upgraded, the way to fix it is as follows:

Go to the rpm database directory:

cd /var/lib/rpm

Create a backup directory for your old database:

mkdir backupdb

Copy all the database files to the folder just created:

cp __db* /var/lib/rpm/backupdb

Remove the old rpm database:

rm -f /var/lib/rpm/__db*

Rebuild the rpm database:

rpm –rebuilddb

I hope this information is useful for you all,
Best regards,
JLChafardet