Try doing a clean install and delete all the cache of the program. Uninstall your software first, then go to the folder location below on your computer then delete the MixPad folders:
C:\ProgramData\NCH Software
C:\Users\(Your computer profile)\AppData\Roaming\NCH Software\[MixPad](%3C___base_url___%3E/forum/69-mixpad/)
If you can't find the AppData folder, set Windows to show hidden files and folders with the steps from this link: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files.
After deleting the MixPad folder, restart your computer then install it again from the next link: https://www.nch.com.au/mixpad/mpsetup.exe
That should do it. Please try it again after that and see if that fixes the issue.