Why does CoCreateInstance return error 0x8007007f "The specified procedure could not be found"?

In Windows 7 I have a COM object in a DLL, let's say it is my_COM_object.dll.

I have two versions of the dll, one is 32 bit and one is 64 bit, the first lives in c:\bin32\ and the second lives in c:\bin64\.

I did regsvr32 c:\bin32\my_COM_object.dll and then I tried to create the 32 bit COM object with CoCreateInstance but the creation failed and the error was 0x8007007f "The specified procedure could not be found".

What can be the cause of the error?

I was in a hurry and I solved the problem with a brute force technique: I deleted all the keys in the registry related to my_COM_object.dll and then I did again regsvr32 c:\bin32\my_COM_object.dll but I would like to know what 0x8007007f means: it does not seem well documented.