I’m assuming you are using a valid email address in the proper format? [email protected] or similar?
The email field is validated by the built-in WordPress is_email() function, that checks for valid email address formats.
All I can tell you is that the plugin is working just fine for everyone else, and for all my test sites.
So, that means that the most likely problem is that some other plugin, or possibly your theme, is somehow causing a conflict with my plugin. Without more information, I can’t tell if those warnings are from my plugin, or again from some other plugin or theme that is causing a conflict. I do NOT get ANY warnings on all the sites I have tested my plugin with.
What you need to do is use a process of elimination to figure out where the conflict is, and then if you let me know which plugin, or theme, is causing the conflict with my plugin, I can try to install it on one of my test sites and see if I can figure out what the issue is.
My suggestion to figure out the conflict is to first deactivate ALL other plugins and then try creating a sign-up sheet. If it works, then you know it’s one of the other plugins, and you can turn them on one at a time and try creating a sheet again until the error returns, and then you know which plugin is causing the problem. If the error still occurs with all other plugins deactivated, then it’s something in your theme. Try switching to a default theme, like Twenty Twelve, and try again. If it works, then you know it’s a theme issue.
If you are still having problems, even after deactivating all other plugins and switching to a different theme, then it’s possible something got corrupted when you installed the plugin. At that point I would completely delete all my plugin files and install a fresh copy.
Start with trying to find if it’s a conflict as described above, and let me know what happens.
