I have a few tips to resolve this issue:
1) Check to see if your antivirus is blocking Hitmanpro. If so, keep it whitelisted.
2) Restart your computer in “safe mode with networking” mode and try to register Hitmanpro with the license key there.
3) If this doesn’t work, then disable your antivirus for a while and then try to register it now.
4) If step 3 doesn’t work for you, then I suggest you uninstall the HitmanPro application completely from the computer and then re-download it again and try to register it now with the same key.
Please keep me updated with the results.