Thank you for reporting this. I apologize for any inconvenience! I’m actively looking into the issue and will let you know as soon as I have it resolved.
In the meantime, I would suggest disabling the display of debug information on the frontend. This should prevent the warnings from showing and is a general security best practice to prevent information about your server from being displayed to the public. To disable you can typically add the following line to your wp-config.php.
define( 'WP_DEBUG_DISPLAY', false );You can also learn more about WordPress Debugging.
If you still run into issues, you can install the previous version of the plugin 2.2.10 for now and update to the latest version as soon as a fix is released. The version with the issue is 2.2.11 and was just released last night. To download the previous version you can go to the Sign-up Sheets Plugin Page: Advanced View, scroll down to “Advanced Options” and select the version from the dropdown.
Thanks for your patience. I was able to track down the issue and have released a fix in version 2.2.11.1. If you run into any other issues, please let me know.