It appears that the signup form on a signup page is getting a 500 error when trying to submit a POST request, specifically when the "I want to volunteer" checkbox is checked and the site's default volunteer page does not have a default point person setting configured (ie. "Assign new volunteers to Brian").
Thanks Brian - that makes sense with the current issue with volunteers need to have a point person.
We will let you know here if there are any updates. Please share & encourage others to follow if they have seen this issue as well.