Not sure why Yoast SEO would be breaking it, unless it’s somehow modifying the sign up links. I don’t really use any SEO plugins, so can’t make any recommendations. I don’t even actively use my own plugin on any live sites any more (only on a dev site, or sites for clients where I’ve made custom extensions).
I would just double-check your settings also. In the main settings for this plugin, look to see if you have the “Redirect Sign Ups to Main Page?” option checked. If so, then you need to also make sure you have that page selected in the option right above it (Volunteer Sign Up Page). If you don’t want to redirect, but instead stay on the page where you put those individual sheet shortcodes, then you need to un-check that redirect option (if it was checked).
Then hover over the signup links and, hopefully you have your browser status bar set to show the URL, check the URL it’s trying to go to. That URL should have the URL of the page you have things set for (either the current page, or the main page, depending on your Redirect setting), followed by 3 arguments that should look something like:
?sheet_id=24&task_id=73&date=2019-12-25
Those 3 arguments, when set correctly, will make the signup form appear for that particular sheet_id, task_id, and date
If Yoast is adding other things, or somehow causing those arguments to be lost, then I can’t help you any more than you have already figured out yourself. At that point, you may want to contact the Yoast developers to let them know what’s happening.
