1First response (you can act now)
You have what you need and can start fixing immediately.
Subject
Re: [their issue] - on it, next update [day]
Body
Hi [First name], Thanks for the clear report - I can see how [the specific impact, in their terms] is getting in your way. Here is where it stands: [what you found or did already, one or two sentences]. I am [the concrete next action] now, and you will hear from me by [specific day/time] with either the fix or a precise status. If anything changes on your side before then, just reply to this thread - it comes straight to me. [Your name] [Team], [Company]