I also am experiencing this problem and I am using the correct serial. As well, this isn't the first time this has happened.
I recently got a message that I needed to go to the store and receive a new serial. I did so, selected Valentina Studio Free, proceeded to checkout, clicked finish and got the serial. When I try to put it in I get a message that it is an invalid serial.
I have not used my Studio Pro version for some time (years? version 8.5.5). When I tried to use it, it said my serial had expired. I found my email receipt, and entered that serial. Still no go. So I went to the website, logged in, and downloaded the serials I had registered with you. Still no go!
What can I do to get my old version working? I might consider upgrading, but not until the one I have works! :-)