What causes "The type library importer could not convert the signature for the member" warnings?

I occasionally get a number of warnings in my web app:

Warning 5   The type library importer could not convert the signature for the member 'ADS_OCTET_STRING.lpValue'.    
Warning 6   The type library importer could not convert the signature for the member '__MIDL___MIDL_itf_ads_0000_0002.lpValue'. 
Warning 7   The type library importer could not convert the signature for the member 'ADS_NT_SECURITY_DESCRIPTOR.lpValue'.  
Warning 8   The type library importer could not convert the signature for the member '__MIDL___MIDL_itf_ads_0000_0003.lpValue'. 
---- snip ----
Warning 33  The type library importer could not convert the signature for the member 'tagARRAYDESC.rgbounds'.   
Warning 34  At least one of the arguments for 'ITypeComp.RemoteBind' cannot be marshaled by the runtime marshaler.  Such arguments will therefore be passed as a pointer and may require unsafe code to manipulate.

There doesn't seem to be anything actually wrong, and restarting Visual Studio seems to make them go away. What causes this, and is there any way to fix it short of restarting?