Getting the exact same error. Everything was working fine until I manually deleted a post from postie, then I started getting the error.
Checking for mail manually
Starting mail fetch
Postie is in /nas/wp/www/cluster-3176/domainname/wp-content/plugins/postie/
Cron: On
Alternate Cron: Off
iconv: installed
imap: installed
mbstring: installed
There are 1 messages to process
Error: Could not update post in the database
Mail fetch complete, 1 emails