Sometime you get the Invalid Argument error, when importing customizations from another instance.
The prime reason of this could be you have exported the customization file before published. You can always go back, publish the customization file and export-import again.
That should work most of the time. But did not work for me.
On importing the customization file, I was getting error Failure:<<relationship attrubute name>>: Invalid Argument. So i opened the exported xml file in notepad and searched for the Relationship Attrubute name, which will look something like
<entityrelationship name="<<Attrubute name>>">
In this section, you will have a section similar to the below
<field name="<<field name">> requiredlevel="none" imemode="auto" lookupstyle="single" lookupbrowse="0">
Replace <displaynames />with
<displaynames><displayname description="<<Any description>>" languagecode="1033"></displaynames>
Now save the file and import again. It should work this time....
Also read the article on same issue http://mscrmuk.blogspot.com/2008/12/invalid-argument-error-when-importing.html .
Hope this helps...