Although you are not sure, you may deliver the message in a positive manner.
Dear valued customer,
We have been working deligently yesterday on finding the solution to this problem. We pin-pointed the root course in the afternoon, and developed the fix. I have tested the patch in the development environment. Let me doubles check with my team lead to make sure the fix is properly loaded in your system, will let you know ASAP if it is ready for you to test.
My team is working very hard to make your testing experience incredible[1]. Thanks for your patience.
[1] This is a joke from Dilbert comics...http://dilbert.com/strips/comic/2004-01-17/
