Hello,
First of all, I suggest you to check current Plesk package and Plesk database version:
# rpm -q psa
# cat /usr/local/psa/version
# mysql -uadmin -p`cat /etc/psa/.psa.shadow ` psa -e "select * from misc where param='version'"
Normally, you have to see 8.3 version. If it is not so, it means that upgrade was failed for some reasons, check Plesk autoinstaller log to get exact cause:
# less /tmp/autoinstaller3.log
If everything looks fine, reconfigure Webmail virtual host:
# /usr/local/psa/admin/sbin/websrvmng -a -v
Hope, it will help you to eliminate this issue.