I was able to replicate your problem. I was able to make a post with other apps successfully.
Based on the error message, either Interstellar is sending an incorrect post type, or piefed is doing some schema checking beyond the officially documented api. I won't be able to look at it until tomorrow probably, so perhaps @rimu@piefed.social or @jwr1@kbin.earth will be able to look into it sooner.
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