Server is 2012 R2. ACT version is 19.2
After installing KB4486105, which is .NET 4.8, our Act Premium for Web website stopped working with this error.
Upon checking recent updates, I saw KB4486105
So, I created a replica of our ACT server and tested uninstalling this update. And boom, it's working again.
You can see in the screen shots above the before/after version of installed .NET version