The old version may be still used when moving between 1.0+ sites.
however when moving from 1.0+ site to a 1.5+ site the newer versions should be used.

Changes have been made so content may be moved from non UTF8 sites over to the new 1.5+ setup.

That said I see no reason why the "compile for 1.5+" button should be trying to download the index2.php.

Does the same thing happen when compiling for 1.0+?

Has your host updated the php version on your server?

I would suggest that you contact your host and see if they have a solution as this can be caused by PHP not being correctly setup on the server.