Keeps asking for serial number

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Mac 10.8.5, Acrobat 10.1.1. About every 5th time I launch Acrobat it asks for a serial number. Why, how to fix?
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It was asking for a serial number after less than three months of use.  It gave the option to input a new serial number or start a 30 day free trial, which was very strange to me.  There were no error messages...just seemed like the original subscription expired, as if I had been using it for a whole year and needed a renewal.
At any rate, I contacted the outfit I purchased the product from online, and they issued me a new serial number that works.  I guess we'll see if this happens again, but it's kind of fishy.
Z