There is a problem with the ifttt service in that an action to add a task to RTM has stopped working. I reported this to ifttt and they said there is a problem at both ends:

Thanks for reaching out! We see these failures on our end as well with Remember The Milk. We will be contacting the team at RTM as the error responses are coming from their API. I would appreciate it if you can also mention it to their support team.
TTThanks,
Art from IFTTT