Until recently we used an external tool to help track links in our email. It appends the link to include "t=a" at the end of the link. Recently, we've been getting a burst pipe when we append this to our custom domain or to our NationBuilder URL.
Kajute, this is now fixed. If you are having any more issues, please email email@example.com.
Thanks for reporting this. At the moment, we are not aware of any use-cases outside of a 3rd party mailer that require the t=a string, which makes the case difficult to escalate outside of our current engineering priorities. However, given that this particular string was working before, but the functionality regressed on you, we can look into why this change happened. That said, the 500 error should not occur on non-custom domain pages, ie customsite-slug.nationbuilder.com/about?t=a.