To me, that implies that those DLL's haven't been updated to take advantage of the increased options, stability and security that has been introduced in later versions of mIRC.
You should check if those DLL's have updated versions, and you simply are using older versions of the DLL because the older version works with your older version of mIRC, but the older versions don't work with the newer mIRC.
Shortly after 6.2 came out, a number of 3rd party DLL's stopped working due to the increased security features of mIRC 6.2.
Since then, a number (if not all) of those DLL's have had updates released.