Virtualmin – not able to add accounts / sub-servers after 10 domains have been added. Check if it is / if it was on a pro license. The pro license only offers 10 accounts, so the system is still thinking you are on pro and hence not able to add.

How to fix this ? or how do you upgrade the license ?

As mentioned, just install the specific version in order to downgrade.

Do the following:

apt-cache madison webmin-virtual-server

Pick the latest gpl package in the list, and copy the full version string, then run the following:

apt-get install webmin-virtual-server=6.0.11.gpl

For example, where 6.0.11.gpl should be replaced with the latest gpl version.