If you open an old MHX rig in the latest MHX version, all panels contain only a single button named "Update MHX Drivers". What is going on?
or the left and right arms are controlled by custom properties called MhaArmIk_L and MhaArmIk_R, respectively. We see in the Custom Properties list that IK is enabled and that they are RNA properties, because "API Defined" is written to the right of the properties.
MhaArmIk_L, but that does not exist because the MHX RTS add-on is disabled.
To fix the file, we have to enable the MHX add-on again. The mesh now jumps into place because the RNA properties are defined again, but this is not a permanent solution.
Now save the blend file, disable the MHX add-on, restart Blender and open the file. The IK pose is still there, even though the MHX tab is absent.