just fixed that for a customer and wanted to let you know about my findings:
First of all, stop postfix, check if it is still running, kill it if necessary and well here are the shell commands

• systemctl stop postfix (old: /etc/init.d/postfix stop)
• killall -9 postdrop
• chgrp -R postdrop /var/spool/postfix/public
• chgrp -R postdrop /var/spool/postfix/maildrop/
• postfix check
• systemctl start postfix (old: /etc/init.d/postfix start)

If that doesn´t help, you might also want to try:

• /etc/postfix/post-install upgrade-permissions
• /usr/sbin/postfix set-permissions 

Hope it helps!