IMAQdx error DLL could not be found

I had trouble with this error message. Fortunately I got it resolved. The solution is actually so simple, but I want to bring it up to the NI community, because there seems to be an issue with the uninstallation of IMAQdx driver. Here is what happened to me. 

I upgraded the device driver to 2019, but something was wrong that I had to reverse back to the previously installed version. So I uninstalled all the Vision acquisition drivers, IMAQ, IMAQdx, IMAQ IO, all of them from the Package Manager. After installation, there is none left in the Package Manager. I was assuming all drivers files had been deleted, but it turned out not to be the case. 

I reinstalled the previous version of Vision Acquisition package, and started MAX. All the cameras showed up, but when I clicked on one of the GigE cameras, it popped a message, saying 'IMAQdx error, DLL could not be found'. Reboot again, still the same message. What the hell ...

By random luck, I checked the 'niimaqdx.dll' file in the System32 and SystemWow64 folders. I was surprised to find that they were still the version of 2019, NOT the version that I reinstalled. That means the uninstallation does not delete the dll file from the Windows system. 

I deleted 'niimaqdx.dll' from both folders, and installed the Vision Acquisition again. Everything is working fine. 

I am not sure if this is a NI issue or Windows issue. But anyway, I want to put it here so if it happens to you, you can save the time and fix this issue.