If you get the error "(-12) Invalid returned data from license server system":
you are probably trying to use Mathcad Prime 6.0 with the old FLEXlm that you can install from the Mathcad 15.0 installation media:
To get floating licenses for Prime 6.0 to work you need to upgrade to a newer Flex. The most resent version you can download from PTC is at the moment (Oct 2019) 184.108.40.206.
So go to support.ptc.com and log in with your PTC account. If you don't have any PTC account you can create it there. If you don't want to do that you can contact us and we will provide you with what you need.
After login you can scroll down and click on the 'Download Software' button:
The expand the 'Additional Downloadable Software Updates' and click on 'PTC License Server: FLEXnet Publisher':
Scroll down and click on the 'English' button:
The you should see the pdf that explains which versions you can choose between. So if your server is using a 64 bit Windows OS we recommend downloading 'Lmadmin 220.127.116.11 Installer 64 Bit' version:
Unzip it and run the setup.exe.
When you get to the third screen, you need your license file:
Don't use the upper part. Just drag-and-drop (or browse for) your license file in the Source field:
Then you only have to wait until you'll see 'Available' as the Status.