This is a business light fare as per EF.
The rules also seem consistent with your purchase. Of course, there might be other things that could restrict it but so far it looks like a valid ticketing.
EDIT: What does give me some concern here is that EUR 1791 fare shown by EF, and your paid value of EUR 1800 is too close. I might be wrong, and I cannot recollect this clearly, but the fare shown on EF doesn't typically include surcharges and taxes. This should put the paid fare higher, but I have also gotten lower fares on AF/KL websites than what EF would lead me to believe many times. So it could still be valid. While using fare routing on EF, I was able to to get EUR 1964, which is still in your ballpark. Even if you underpaid, that is what the system quoted and issued you tickets with valid fare basis codes - so your ticket should remain valid. An issue of this form happened once to my mom, where the agent collected a lower amount than needed for an
upgrade; but she flew in J and the issue was resolved post flight. That J seat should be yours.
But see quote below.
The section I bolded is the one that might be something that has led to your situation. Maybe the system hit some kind of capacity limitations for this fare and downgraded you?
Either way, if they cannot fix this, you are definitely owed an involuntary downgrade compensation.
Are you flexible to travel one day later? Maybe see if if they can offload in you in Paris and have you fly tomorrow with a seat reinstated in J?
This might be the PSL creating new bookings for you, to try and hold the seats.