First, SW2015 is not officially recognized to run on Win 10 unless it is SP5.
Secondly - before you reinstalled Windows, did you deactivate the SW license? If not, it is possible that the program is still looking for the machine with the older operating system.
Thanks for your quick reaction.
I also had a thought that it would be still activated. I indeed didn't deactivated it before i reinstalled windows.
Is there a way to still deactivate it, or should i contact solidworks about this?