I have a similar problem. Following recent purchase, Office 2011 for Mac (Word, Excel, Powerpoint in particular) repeatedly but not after every load or usage, puts up dialogue box asking for the product key. I can enter the 25-character key now or 'later' (the latter tells me I've activated the product even though the sequence will be repeated), but, as ever, "the activation server is temporarily unavailable".

The product still has at least one of its original five licenses, the product key is correct, the product runs as expected, but the activation nuisance persists. Anybody found a work-around for it ?