Letter claiming money owed + suddenly unverified for Housing on UC Journal

I live in London, UK.

2 weeks ago I received a letter out of the blue, from a council I haven't lived in for 12+ years, regarding a claim for an address I lived in 14+ years ago, and addressed directly to me at my present address.

They claim that they had overpaid me Housing Benefit. I have always been meticulous about paying my rent, ensuring they have all details of wherever I've lived that accurate claims were made. I've never hidden from creditors, have moved 4 times since this address, yet this is the first time I have been contacted about it.

I spoke to a customer service person who was as baffled as I to why I'd been contacted, why this debt even exists, or why it took 14 years to contact me and advised me to email the overpayments dept with any info I could find.

Apparently they sent 1 letter 13 years ago to an address in the same borough that they'd been informed more than a year before, that I had moved out from??

I frantically searched through as much of my old emails and paperwork as I could to find evidence, but as you can imagine 14 years and 4 addresses ago - who keeps 14 years of stuff? especially when everything had been done to the letter, without any problems.

Last Friday I checked my bank account and saw my housing benefits for my current address had been removed from my claim.

I've lived here for 4 months (different to the overpayment letter borough).

As soon as I moved, I uploaded my evidence of new tenancy agreement, landlord's details, utility bills etc and they were all verified on the journal.

My claim was smoothly adapted and transitioned over to my new now current address.

Again out of the blue, after checking my bank balance this morning before paying my rent, I saw that the amount was VERY short. So I check my universal credit journal which now claims my address HADN'T been verified and that they were clawing back the past 4 months housing amounts, changing the past 4 months statements.

I am overwhelmed with anxiety, obviously I can't do anything till someone contacts me for either of those things, haven't heard back from the previous borough overpayment email I sent and I have absolutely no idea why both of these things are happening, let alone why they're happening all at once.

If anyone can shed some light, I would be most grateful