UPDATE :

Greetings.
Last night ( 30.06.2021 ) I did a factory reset on my device, and 30 minutes later I tried to use my balance in Google Play again, to buy certain offer in T11... without success...
Today, when I made my morning coffee, read the news and all the regular daily stuff, decided to check my team, do some daily training... and guess what happened ?
The "Mourinho's magical fingers" touched my shoulder... transaction success !
No explanation can be provided, as I did nothing new from the last night's try.
I can't explain how and why... it just worked today.
If, by any reason, someone of the devs was in my account during that time, and fixed the problem, then I'd like to thank you for the support.
Case closed, and I hope no future topics of this kind will occur