OHMS Import Plugin LogicException

I’m working on setting up a small oral history website for my university using OHMS integration, but I can’t seem to make the OHMS Import plugin play nicely with Omeka. I followed the installation instructions and uploaded my test .xml files to the cachefiles directory, but I get the following error when importing the zipped .xml files via the import plugin:

LogicException: OHMS file must be validated before retrieving the list of columns. in /var/www/html/plugins/OhmsImport/models/OhmsImport/File.php:48

Tried reinstalling the plugins, changing the OhmsImport dir permissions to no effect… Interestingly, the OHMS viewer works fine when entering the proper URL on our server. Any ideas what might be causing this error with the import plugin?