I was going to suggest trying to run mIRC in XP or Vista compatibility mode, but I'm not sure that works. My original question was due to the error message you report being ambiguous, since it's the same error you'd see from:
//echo -a $dll(file_does_not_exist.dll,abcd,efgh)
This dll gives the same error on 32bit Win7. The fact that it gives this error when I give an invalid function name - tells me that mIRC doesn't even get as far as trying to call the function name. Perhaps its LoadDll function is doing incorrect which is no longer forgiven. I have an even older DLL which provided support for $md5 hash digest and it still works, so the age in itself doesn't look like it's the issue.
