Solved ! (Thanks Ayesha !)
TurboTax recalculates the maximum transferrable amounts using the data under "Other Dependent Data". Get something wrong there and the calculation is incorrect. (You have to add up a wide number of lines from the student's return, instead of just transferring one line number).
Actually even get it right and the calculation still seems to be incorrect. My students return had < Max federal and 0$ provincial available, but my return said both had max available.
The error message was useless for finding this issue.
It would be FAR more intuitively obvious if this information was requested under the dependent page instead of somewhere else.
