Error adding signup record. Please try again.

Everything is still working fine on two different development/test sites, even with the new WordPress 4.0.

I can’t reproduce the problem, so it is most likely something with your site. Have you added any new plugins or changed themes? Did you update something? Did you just do the update to WordPress 4.0? That shouldn’t break my plugin (I just did it right now, and it worked fine after the upgrade), but it could have broken some other plugin or something in your theme which is now causing conflicts with my plugin.

Possible database corruption or some other issue. It’s very hard for me to know what’s going on without a lot more details. The plugin itself is not broken, at least not on the two sites of mine that I just checked it on.

The only times that error message should appear is if either WordPress returns an error when trying to add the signup to the database, or if you enabled quantities for the task but the quantity the person is trying to enter is larger than the # remaining (although that should be caught by the signup form validation before the plugin even tries to write the signup info to the database).