Thank you for starting your own new topic. ;)
It works on your installation but not on the clients. One of the differences is that you're running Genesis and the client is running Thesis.
See if you can have the client try these steps working backwards just as a means to identify the source of the problem.
On the problem installation:
- Deactivate all of the plugins except the Tweet Old Post plugin and see if you can reproduce the problem.
- If that does not work, switch the problem installation to the Twenty Eleven theme.
- If you haven't already done so, check the plugin FAQ page.
When you use the plugin on the clients blog, do you get any error messages or blank pages?