Signup links not working since last upgrade

I have been setting up the above site with PTA Volunteer Sign Up Sheets and it was working perfectly. Now that I’m ready to take the site live, the signup links aren’t working. This seems to have been since the last upgrade. I noticed every signup URL ends in ‘signup_id=0.’ I have tried deactivating all plugins. I have also moved the pages with the PTA shortcodes to different levels in the site. I have also tried creating new signups. All to no avail.

I would appreciate any thoughts you might have. Thanks!