The issue:
"An error has occured - You have reached the maximum number of free on-blockchain transactions for today. Do you wish to add a 0.0001 BTC fee to proceed? (This fee does not go to Coinbase.)"
The answer from Coinbase:
Thanks for contacting us about this, and my apologies for the inconvenience. This error will show if your account sends more than 25 transactions a day. This is a recent change which explains why you had not seen the error before. Moving forward, transaction fees will have to be added after 25 transactions per day.
I hope this is not too much of an inconvenience. If you need assistance with this or further clarification, just let us know.