Thanks for the clarification. I will read the manual and it is good that you pointed to the naming issue. Also thanks for all the efforts you guys did in developing the processor and I am sure the atmospheric correction algorithm does a great job.
However, in my opinion the so-called 'Windows problem' should be resolved properly. You can imagine that the majority of users install SNAP in a Windows environment. In my earlier experience with BEAM in Windows, the atmospheric correction plugins worked seamless for the older MERIS or CHRIS images (no additional software installment or renaming required). I was expecting the same experience for S2 in SNAP but it appeared to be way more complicated. Both in Windows 7 and 10, processing the images caused error messages.
Although manually shortening the image name may somehow resolve the issue, however, the user cannot shorten to just any name but, as you mentioned above, needs "S2A_OPER_LIC". The user should be made aware of that (without reading this forum first).
I will now try the work-around as proposed in the Manual (point to a Network drive), but I believe a lot of trouble can be avoided if informative error messages would be provided with suggestions for solutions. Even better would be if the software first checks the name, and if too long, e.g. uses internally a temporal shortened name for further processing.
Thanks again for all efforts. I will now finally start using the images.