This happened to me also. I bought a X-Plane key from G2A, and used it for over half a year until one day it just suddenly stopped working.
I Tried contacting the person who sold me the key via G2A (Eche87685), but He/She just ignores me.
I asked for help from Laminar Research, but they just told me that "my key had been either bought using a stolen credit card or the original purchaser deactivated it for money", and give me a link where to buy a "legit key" :(
I asked for help from G2A's own support, they just tell me to contact the seller (who doesn't answer to me).
I tried claiming a refund through PayPal, but I just got an automatic response that my claim has been denied, because it has been over 180 days since the transaction happened.
If I had bought the key using credit card, I could have had one more chance to get my money back. But at this point it looks like I lost my 27.71€ for good :(
So I would recommend NOT to buy X-Plane keys from G2A, or any of these "marketplace sites".
Did you buy from the same seller "Eche87685"? Did you get your money back?