Yeah there are already some topics with the missing Interrail reservation option at the website of SNCF

Only way is actual a International Ticket desk (but even here are many domestic trains within France not possible anymore) or you call the SNCF Hotline for reservations :)

I live in the Netherlands and at the ticket desks they charge €7,50 for a reservation. Do you think it's still possible if I do it the day before in Luxembourg?

AFAIK Luxembourg - Paris was never possible online and has to be done at the ticket office. For the night train Paris - Nice, I'm afraid you'll also have to go to the ticket office. You can do this in the Netherlands but it will cost a fee of 7.50 per booking (up to 22.50 for small groups):

https://www.nsinternational.com/nl/waar-treintickets-kopen/verkooppunten-op-ns-stations

(One booking can contain multiple reservations for multiple people.)

In Belgium, there shouldn't be an extra fee at the ticket office. In France, you can do it at the ticket office or ticket machine (also without any extra fee).

Please complain to SNCF that you can't book Interrail reservations online anymore in/from/to France. If enough people complain (mail: customer.service@oui.sncf), then maybe they see it as a problem and fix it.

That's actually a bit risky because on the international TGVs the available quota for Interrail is limited and can sell out before the train is full.

SNCF do a good job of hiding the telephone number for reservations but maybe customer service can put you through: +33 (0) 1 84 90 90 90.