It's been a while, but I seem to recall that the issue was about HU using a manpower agency to hire rotational pilots/engineers, as contractors, for overseas contracts, thereby avoiding tax liabilities etc in France. When a pilot/engineer (or more than one) were 'let go' they went to court to lodge an unfair dismissal claim and HU tried to say the staff were not employees. The court not only disagreed but said they were also guilty of tax avoidance, and fined them - this would appear to be borne out in the wording of the failed appeal judgement.
Purely from memory and may not be quite right. It came up when doing Due Diligence on HU in 2015.