Post adaptive card to Teams Channel stopped working

[KeithCully]

Between Friday 1st of April and Monday 4th of April a breaking change was made to Power Automate which is causing existing Flows using the "Post your own adaptive card as the Flow bot to a channel" action to fail.

The flows are failing with the below error, even though there has been no change to the flow and there is no date time parameter/ option for the action:

Unable to process template language expressions in action 'Post_your_own_adaptive_card_as_the_Flow_bot_to_a_channel' inputs at line '0' and column '0': 'In function 'formatDateTime', the value provided for date time string '' was not valid. The datetime string must match ISO 8601 format.'.

[KeithCully]

Addendum: This also reproduces in the more recent "Post adaptive card in a chat or channel" action.

[KeithCully · Accepted solution]

It turns out that a separate upstream dependency had a format change that caused an invalid value to be processed.
This is not a Power Automate bug