When Retrieving MEP content an error occurs. This is caused by a bad heads in the HydraCAD for AutoCAD project. 

Command Failure for External Command

When Opening HydraCAD or trying to list a drawing, you recieve any Error containing the following CLSID:

44D62FAE-67A7-4131-BF38-489CDFE44738

Or any of the following HRESULT error numbers:

0x800736B1

0x80040154

0x80070002

0x80080005

0x80029C4A TYPE_E_CANTLOADLIBRARY

Double-clicking your HydraCALC icon makes nothing happen.  You do not see HydraCALC open onscreen nor on your taskbar.  HydraCALC does not open other than in the task manager.

If you open your task manager and see the app "Hyd_prog.exe" open but not on your taskbar, then the application is stuck in purgatory.

Another possibility may be that it's on the taskbar but not on your screen.  That means it's open but offscreen. 

See this HydraCARD to fix that issue;  HydraCARD 565 only AFTER you've checked on the solution below.

Blacked out cells in Head Selector dialog

Calc process stops at step 1a.4 and Runtime Error 5 displays

Could not find file, or Unrecognized database format 'C:\HES\HydraCAD\Ver50\AutoListElements\Data\Setup\mdbtrans.mdb'

When trying to open Add Device, nothing happens and Revit just hangs up / appears frozen.

No error, but after clicking Setup, Revit freezes / stops working, and nothing else is able to be selected.

When selecting "Retrieve Part Num/Desc <" from the sprinkler Block Definition window, you get the error;

Error: bad argument type: stringp nil

When attempting to insert a pipe dimension you get the error "Invalid option keyword"

And the command prompt says "Enter an UNDO control option" with the dialog showing your choices