Marketo LPs haven't needed the ".html" for a long time now. We advise clients to skip the extension, so this isn't a new thing.
FTR, Too many redirects is a client-side (browser) error, not a server error. It occurs if you have page1 → page2 → page3 → page4 ..., but is even more likely to happen if you have an infinite loop page1 → page2 → page1 → page2 .... (An infinite loop will always exceed the "too many" cap!)
So I don't know exactly what happened here, but I'd be 99.999% sure your instance was already able to use the ".html"-less URLs before this weekend.
It may also have to do with instance age, location and setup.
We had no ability to not use .html until this happened. It may also have to do with the servers or PHP... Maybey something do to with how the page url is stored, posted, and redirected. I am thinking if they were both available that they were treated as independent pages. With recent updates the page urls are referenced based on their core url name (sans html - like how we assign new url name to the LP Asset). Now regardless of html being included in the redirect, the 301 only only took into account the value between the slash and html and applies that redirect to both url variants. If the redirect doc was tweaked somehow to look at that, it would definitely cause a looped redirect. I'm no dns/php/web whiz kid, but this is my feeling about what happened.
Besides we are hosted down in Sydney... Things get a little funny once you cross the equator