If, after you upgrade (and redeploy) to PS 2020, your Powerserver web application won’t start – or if your windows won't open or if they do and most of the datawindows don't display, then PANIC. 

Actually, don't panic as it is most likely that you have 10X compression turned on which REQUIRES your f files to be mapped in IIS.  Wait you say, i recall doing that already!  Well the upgrade will clear that mapping for some reason, so you have to redo it.  

Also, Make sure you clicked module mapping and not module handler.  The screens are IDENTITCAL except module mapping has a place for an optional executable.  I have selected the wrong thing many times.  Just clear that mapping and recreate it correctly.  The required settings are listed in the powerserver post install instructions.

The instructions state that you must restart IIS for the mapping to 'take'.  This is not true for more current versions of IIS, but you should probably just restart anyway...

Your other option is to clear the 10X compression settings, which means your app will download slowly