Recently Received Barking Complaint, no details just "nuisance"

I have two puppies that are my entire world, they're both almost 1 and have never been dogs to bark a lot. Usually, one of my family members is always home and sorts out the issue that is making them bark – no water, holes, etc. Just today I've received a letter from the council informing me someone filed a complaint of the barking being a nuisance.

For context the puppies are both in bed by 9pm, and do not bark at night. They bark sometimes when playing with each other (hard to stop), and when excited which quickly calms down. No neighbour has come to my door asking in any way to keep the dogs from barking. No one has sent a letter, left a note, nothing.

I have no idea which neighbour complained, or what exactly prompted them to send the complaint since I've not gotten one since I've had them. I'm worried that I'll lose the pups because this person quickly escalated to council complaint without even confronting me first. There are multiple dogs on my street that also bark, even a house that blasts music late into the night.

Any advice? These pups are my world and mental support, I really can't lose them. (I'm willing to sort it out with the complainer, but I don't know who it is or why they complained specifically since it was issued 10 days ago and I just got the letter!)

Edit: I didn't expect so much advice so fast, thank you! I'd like to note I have looked up the council laws in my area and do have cameras around my backyard and at my door. My council has it as you must converse with the offending neighbour before complaining (I've proof they haven't – no letter, no knock on door.) So, I'll call the council and let them know, as well as start noting all the times my dogs bark with reasons and recordings.