If, like I did recently, you export an entity before it has been published, you will get an 'Invalid Argument' error when importing the entity if it has any relationships.
The reason for this is that the relationship attribute will have no display name, which results in the error. The simplest solution would be to go back to the original deployment, publish the entity, then export again. But, if that's not an option, you can fix the problem within the exported xml as follows:
The invalid relationship xml will look like the following:
<field name="pjv_targetid" requiredlevel="required" imemode="auto" lookupstyle="single" lookupbrowse="0">
The problem is that the displaynames element is blank. This can be changed to something like the following
<displayname description="Target" languagecode="1033" />
The languagecode attribute will need to be set to the correct value for your deployment. Many other elements will have this value set in the customizations.xml file