Booster license for 3 computers: Serial no. / licence error at computer 2+3

I bought a licence for 3 computers (Reference  no.  xxxxxx). Installation and licensing at 1st PC = ok. Installation at PC no. 2 + 3 = ok, but Licensing failed (error, contact support...). Any idea? Probably I got a single license or ...?

Failure message (translated from german): "Unknown error. Please contact F-Secure customer support".

Of couse I did, however, maybe somebody knows a simple solution that I couldn't imagine...

PC no.1 = Windows 7

PC no. 2 + 3 = Windows XP SP3

(if it is relevant)

Thanks.

EDIT: Removed PII