Hi robinv ,
FMS works with specific(s) version of Java. Once that version is installed, you should not update Java.
Here FileMaker Server and Java - Overview | FileMaker you will find the required version for FMS 12 and others.
You can check the actual Java version by going to the Control Panel and then clicking on Java, going to the Java Tab and clicking the button. If you don't have the required version you may install it.