I have spent the past few days banging my head against a wall trying to figure out why the WordPress installations on some servers continually generate "Failed to import" and "Invalid post type" errors when trying to import a theme .xml file using this Importer plugin. In addition to the failures the .xml file was also not recognizing there were any authors in the file.
Originally I had thought it might be an issue with a missing module/extension server side. I ended up testing this Importer in identical environments across different servers (shared, dedicated, vps, etc.) all with mixed results; sometimes it worked other times it didn't. I tried all manner of php.ini memory settings etc. with no luck. At this point I'm still not sure if it's server side or the plugin itself behaving badly.
I FINALLY got the data to import and am posting my method for others to try to see if it helps them; not sure why it requires this process but importing the .xml would NOT work any way I tried on some servers until I used this method.
If your .xml data import is failing try the following
(this method resolved this issue for me):
1) Download and Install 7zip.
2) Right click on your .XML file -> 7zip -> Add to Archive
3) Change the "Archive format" to gzip and hit "OK". You'll now see your file with a .gz extension on it.
4) Try to import/upload the file again (selecting the .gz you just created not the .xml file) and it should work.
The first 2 times I tried this method I actually hit another error. Step 2 of the import ended up going to a "404 not found" page on the front end. To get around this I had to bump up the following settings in my php.ini:
default_socket_timeout = 600
pfpro.defaulttimeout = 600