All of a sudden the SP_Send_Alert_Info token in alerts is generating an invalid link. The LeadID in the invalid URL is correct. The SFDC link still works.
What part of it is incorrect -- how does it differ from the URL when you navigate to the same person in SFDC Marketo?
which sends us to the dreaded Quoth the Raven - 404 page.
No change was made to the alert.
Hmm. You've gotta open a ticket.
In a few tests just now (on app-sj01, so not identical to your setup) the only way I can force a 404 is if the tok query parameter doesn't exist at all. Even if I put a random tok value in place, and don't include the leadId or panel query param, I get a 200 and a white screen (i.e. an unusable page, but not the Quoth the Raven 404).
If you don't need the temporary link part of SP_Send_Alert_Info, you can of course build the same thing yourself.