Thank you so much.
I realise now I kind of wasted 3+ days trying to figure out what the problem was, researching in Internet and rechecking every corner of my system.
It was simply that.
I uninstalled Java 8, installed java 6u45 and ammended the environment variables to pint to the right paths.
And startup.bat run with no issue and I only had to refresh the browser that was even previously open showing an error.
I can start now the jBilling web ui.
However, when I try to access Trend, for instance, it lets me select only company "-", and admin/123qwe is not good for the selected company ("-", since I cannot chose anything else or type anything in the dropdown box.
What am I doing wrong?
Do I need to do something special to start the Trend database?