Post cache substitution is not compatible with modules in the IIS integrated pipeline that modify the response buffers. Either a native module in the pipeline has modified an HTTP_DATA_CHUNK structure associated with a managed post cache substitution callback, or a managed filter has modified the response.
"set this to false instead of true"
This post is quite old, but I am having same problem with newer version of Sitefinity.
I am having similar problem where it shows this error when I logout from my website. I have another copy of website which has same web.config and I don't see this error. I checked config for dynamicCompressionBeforeCache and it is set to false for both websites (one where I have problem and one where I have not). I compared both web.config files and it does not have any difference.
I am running sitefinity 6.3.
Any help is greatly appreciated.