Since you uninstalled 3.0.3., you can check the DxO log files and the Windows Event Viewer logs (Application and System logs) at the time you tried running 3.0.3. to see if there is any record of what went wrong. It could also be that your download was corrupted.
Could you, please, try to download it again and install? If you still have a problem, please, provide us with the logs ( %UserProfile%\Documents\DxO PhotoLab 3 logs) . You can attach it right here. Without it we can’t investigate the problem.
And one more question, which version of Win10 are you running (I mean 1903 or 1909)?