When trying to report I get a 400 error with the following JSON response
{
"error": "couldnt_create_report"
}
The JSON sent in the request was
{
"auth": "[auth_token]",
"comment_id": [numerical_id],
"reason": "[reason]"
}
which seems about right.
I was able to report this post successfully, so something must be weird about whatever post or comment you're trying to report. Maybe they blocked you?