We now identified the bug that was indeed introduced in version 2.2.5 of the plugin. We didn’t come across it during testing, and as mentioned before we had some difficulty in reproducing the bug.
Once we started testing on WP Engine, it was easy to reproduce and relatively easy to fix.
The bug only seems to occur for WordPress installations that automatically create a thumbnail for PDFs. Not all WordPress installations have this enabled, so that’s why we weren’t able to reproduce it at first. Hosting services like WP Engine (and I’m guessing Liquid Web’s WP platform as well) have this enabled by default so that’s why it was easy to reproduce as soon as we started testing on WP Engine.
We’ve just released version 2.2.6 that fixes the error you were getting with version 2.2.5 of the plugin. I’m currently unable to reproduce the error on WP Engine, and I assume that it would be fixed for other WP hosting services as well.
Please give the new version a try, and let us know the results. Thanks!