ISPmanager License requires to be regularly updated. If you have not done it in time, pay for the renewal and update the license (you do not have reinstall the software). Run the following commands on the server:

For ISPmanager 4:
rm -f /usr/local/ispmgr/etc/ispmgr.lic
wget -O /usr/local/ispmgr/etc/ispmgr.lic http://lic.ispsystem.com/ispmgr.lic?ip=IP_ADDR_YOUR_SERVER
killall ispmgr
For ISPmanager 5:
rm -f /usr/local/mgr5/etc/ispmgr.lic
wget -O /usr/local/mgr5/etc/ispmgr.lic http://lic.ispsystem.com/ispmgr.lic?ip=IP_ADDR_YOUR_SERVER
killall core