Trouble importing content

I am in the process of merge 2 similar wordpress sites. I have successfully exported the content of the “old” site; however, when I try to import “old site” content to “new site”, I get this error: “File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.”

I checked the exported XML file, and it was 20mb. In php.ini, post_max_size and upload_max_filesize are both 100mb.

I repeated the export/import process with the same results.

Any ideas as to what might be causing this error?