I've been running BIAB for awhile. It works fine for awhile, and then I get prompted to activate my account - again - by entering the serial number.

What's up with this?

We see that you had marked this as resolved. But for anyone else who comes across this thread, you will need to activate Band-in-a-Box with your new serial number after installing a new version/year of the program. If you do not activate it right away then you will start seeing prompts to do so after 30 days. You can activate under the Help menu of Band-in-a-Box.