Unfortunately one of the announced bug fixes was not part of QF-Test 4.2.1.
We have immediately corrected this with the now available patch version 4.2.1-p1 (Release Notes)
The issue only applies to some special cases under Windows. There is no need to update for those not experiencing any SUT starting issues with QF-Test 4.2.1.
In case of any questions or doubts, please get in contact with our support.