Inlet liner keeps cracking after installation on Agilent Intuvo 9000 GC coupled with a 7967A headspace sampler and a 5977B MSD

I'm using the 4mm glass wool liner (pn #: 5190-6168)

What liner are you using? Agilent part number ??  A picture of the top of the GC might help.

I'm also receiving a message error "Error code 20401000, cause: the EPC module is going into a shutdown, effect: the GC beeps to alert the user."

this error message occurs during my postrun.

That part number is: Inlet liner, Ultra Inert, splitless, straight, 2 mm id for use with HS Transferline. 

https://www.agilent.com/store/productDetail.jsp?catalogId=5190-6168

Make sure you are running SPLIT mode with a headspace injection and at least 20 ml/min of total flow.  The GC electronic pressure control modules need at least 15 to 20 ml/min of total flow for reasonable control.

Do you have a Post Run time setup in your method?  GC Oven page, Post Run Time. 

  If you have a post-run time there with a post run temperature, the system will also try to drive to the Post Run flow on the Columns page.