Suddenly - starting on 1st July - the region Sud, aka PACA, has decided it's not accepting national and international rail passes and discount cards on its TER network. Judging by the reactions, this was not made public in advance. Whether it's just a fit of devolutionary zeal, or represents some other political posturing, is not clear.
Interrail offer this advice:
Updated 4 July, 2022
It has come to our attention that the transport regional authority in the “Sud PACA” region of France (Sud Provence- Alpes- Côte D’Azur) is currently not accepting Interrail/Eurail Passes on board TER (regional) trains. We are investigating the matter directly with SNCF▸ (France's national state-owned railway carrier and Eurail B.V. shareholder).
We want to make sure that this development impacts your plans as little as possible, so for the timebeing, please purchase a full-fare ticket in order to continue your journey using TER trains. We will refund the costs incurred to make this possible. In case you received a fine on board, we will also refund those costs. Please keep as much documentation as possible to submit together with your claim. This includes receipts for tickets or fines, the number of the train you were riding, and the date when the fine occurred.
Please keep in mind that refunds can take up to 6 weeks to process. We thank you for your patience and apologise for any inconvenience this may cause.
SNCF don't seem to have been prepared either, so websites have not reacted yet. In one place on twitter, they said TER Occitanie was affected too, but elsewhere that's not mentioned so it's probably not the case.
I think that tickets bought before July at a discount will be honoured, and a pass bought in advance ... maybe. It's hard to say as the only SNCF info I've seen refers to the date when a ticket was bought using the pass, so it isn't clear how that applies to a pass used in lieu of a ticket.
