ISAPI DLL stops working in 10.1 Berlin

Hi!
I upgraded to Delphi 10.1 Update 2 and now I get an error:

"Received content of invalid Content-Type setting: text/html - SOAP expects "text/xml"

when calling just ANY method of my existing ISAPI DLL. Everything worked fine in 10 Seattle with an x64 build target and IIS 8.5 that comes with MS Windows Server 2012.

To get an idea og whats going wrong a created a new ISAPI DLL with 10.1 from scratch, including the echoXY example methods. I called them by a very simple VCL Forms client that just contains a Button and the HTTPRIO component. The error stays the same. So ISAPI seems to be broken in 10.1. I did not change any single setting on the server.

Is there any workaryound? I spent many hours googling and found a number of solutions that did not work like skipping the ISAPThreadPool unit and so on.

Thanks, Reg