Problem
Classic, Responsive, and New Responsive forms may not open or save when they're published on a list within a site that contains an ampersand (&) in the site URL.
For example: https://tenant.sharepoint.com/sites/research&development
Solution
This is a known issue. The behavior of each Nintex form type varies between the SharePoint Online Classic and Modern experiences. Each is documented below:
Classic Form in Modern Experience: Opens but will not save; an 'Invalid URL value. . .' error will occur
Classic Form in Classic Experience: Opens and saves as expected
Responsive Form in Modern Experience: Opens but will not save; an 'Invalid URL value. . .' error will occur
Responsive Form in Classic Experience: Opens and saves as expected
New Responsive Form in Modern Experience: Form will not open, redirected to a page stating 'Page not found'
New Responsive Form in Classic Experience: Form will not open, redirected to a page stating 'Page not found'