Lightroom 6 serial problem

HEllo,

I purchased the LR5 to LR6 update a few days ago.

Managed to install it on my Macbook and activate the SN without any problem.

I'mve been trying to instll it on my iMac for 2 days now and I have a message that my SN can not be validated by Adobe.

Does anyone have a clue ?

Thanks !