The image on the right shows an example of what was meant to be a duration token, but where the "m" was omitted.
While this message is not necessarily caused by a malformed duration modifier, what is always true is that it is issued at a stage where no further processing options are being processed for the current holidays token, and therefore any part of the token after that colon will be ignored (and therefore might as well be removed to avoid confusion).
Topic 185555, last updated on 29-Mar-2021