Export/Import XMI 2.1 fails

[purplecat7]

Hi all,
EA version 13.5 running on both Windows 10 and CentOS (under Wine).Packages version controlled with SVN.
Scenario:Colleague checked out a bunch of packages and made lots of changes, but checkin does not work. i.e. there's a progress window but nothing to ask for a comment, and the svn repo and model both indicate it's still checked out.Logging in as him, I can checkout and undo checkout of a package. To save his work, I have exported every package to file with the XMI 2.1 option.I've then undone the checkout, made sure everything's clear and in sync, and tried again.
But it's all gone a bit wrong...
Things I've tried:

• Import to a new package.
• Merge direct
• Merge with file
• Import to new model
• Hand crafting the xmi:version=  in the file
• investigating 2.1 -> 1.1 xmi converters (no luck)

With the imports I get:

• Package import caused error: DAO.Recordset [3058] Index or primary key can't contain a Null value.

With the merges I get something like:

• Invalid EA XMI 1.1 file

[Can't see anything in the doc about 2.1 NOT being allowed for merges]
I have, of course, been all over the web/forums for several hours now.I've been using EA since it came out so am not a novice, but this one has me beaten.Not looking forward to telling my colleague that he'll have to redo all his work 
Help!!!