Yes, the developer, Aditja Jain, appears to have disappeared off the internet which is a shame, as his plugin is/was fantastic!
The problems seemed to arise after updating to 10.0.6 which threw up an error message on self-hosted websites, including my sites.
However, the quick answer was to uninstall and re-upload the version 10.0.3 which I was originally using and NOT to update the plugin to any other version!
I am currently successfully using 10.0.3 with v3.5 WordPress on a 'Subscribe' page within which the form works from.
If anyone would needs a copy of the v10.0.3 plugin please PM on here or contact me via:
Hope this post helps somebody.