BizTalk 2006 : can't create instance error (XLANG/S)

If you get error like:

Could not load type "B2BSchemas.PO_Num" from assembly "B2BSchemas, Version=1.0.0.0, Culture=neutral, PublicKeyToken=d7b72bf643e25262".

Check that:

1. Your assembly is correctly deployed in BizTalk (i.e. check in Resources) - look at the version and ID
2. Your assembly is in GAC (again version & ID)

If both are ok, BizTalk is "messed up" and you need to restart the host instance (ie. Platform Settings > Host Instances, select the instance you're using and right-click > Restart).