You can't contact Adobe because they don't support Lightroom 6 at all anymore. It's out of production, ancient history, not on the Adobe radar at all. You should have your serial number stored somewhere. Check your account to see if it's registered in your account information. That serial number, which is 24 digits, the first four being 1160, is what you are going to need in order to activate your Lightroom.
however, when trying to actvate the license on my old PC with the new motherboard, I have got a message that my maximum number of installations has been made already; obvioulsy the old motherboard is still counted; I need to 'release' this somehow as I still work on my same old PC, just with an new motherboard -therefor I need advise how to do this
You have to contact Adobe and tell them you want to deactive computer installations of LR 6. There are plenty of examples here in this forum of people doing this successfully. Even though LR 6 is "unsupported", Adobe has deactivate installations of LR 6 for many people, including recently.
I have never had Lightroom 6. But I believe you can go to your account information and find a list of active devices and deactivate a device. After doing so that will enable you to activate a "new" device even though it is the same one.