In Response To 'Re: Re: A link to the Java runtime could not b...'
I finally managed to trace down the problem to a utility program called tuneup utilities that has the ability to temporarily deactivate programs and reactivate them when needed. Even though I hadn't deactivated any Java program, some part of the programme must have interfered with Mathematica. I uninstalled tuneup and reinstalled it afterwards and now everything works.
Unfortunately I could not trace what particular service or program has caused the initial hiccup.