This is not the support forum for Sign Up sheets, although many people are trying to turn it into that. Also, this particular issue is a server configuration problem, and has already been brought up and answered on the support forum for this plugin at WordPress.org. See my final reply in the thread here:
https://wordpress.org/support/topic/not-able-to-add-new-task/page/2/?replies=17
I would suggest you change your PHP configuration to allow more PHP max_input_vars, or, as a more simple solution, since this is a weekend thing, make one sheet for Saturday and one for Sunday, thus splitting up the tasks for 2 days. Multiple sheets can be shown on the same page now by using more than one shortcode.