The serial number is already used by the maximum allowed computers

My Adobe Acrobat XI Pro worked fine, when - probably due to a malicious program that might have removed my registration -  it started to say that I have reached the maximum number of allowed computers it can be installed on and I will have to provide a new serial number within ... 10, 9, 8 ... days.

I was kindly assisted by Adobe Customer Service and was asked to uninstall and reinstall again, since the Deactivate menu line was dimmed.