Problem with licence

I've a regular licence for home use.
I was using the client on Windows 8 but I had a problem and I'd to downgrade to Windows 7 ( the problem was related not to SmartFTP) and now when I install the licence key the client returns "License error: License exhausted, cannot allow another instance."

I can't request suppor troght the client becouse the licence is not present.

I've to buy another licence?