Activation problem with Trados Studio 2017

Dear all,
I'm currently trying to upgrade my SDL Trados Studio 2015 Freelance version to SDL Trados Studio 2017. Unfortunately, I cannot activate SDL Trados Studio 2017. I get a error message that says "Please provide quantity less than or equal the remaining quantity 0". It isn't possible to return the license/force return neither. Despite the fact that I've deactivated SDL Trados Studio 2015 before installing and trying to activate SDL Trados Studio 2017, could this activation problem be related to my SDL Trados Studio 2015 being still installed? Could you please help me with this issue?
Thank you for your answer in advance.
Best regards,
Eva-Maria Schwarzböck