Very strange letter about incapacity benefit but I have NEVER claimed it!? Help!

I have recently become self employed alongside my employment and set up for this with HMRC. I got a letter today that acknowledged this start date but then said that their records show that I made a claim for Incapacity Benefit on 6th April 2022, and that the claim is still open. I have never in my life made a claim for anything remotely like this and have been working since 2014. My only reason that I belong here is for my son’s PIP claim as I am his appointee. I have never claimed anything like this or had any examination or medical.

Several things don’t seem right about this letter. There are no phone numbers at all on it to contact, the gov.uk website is just that with no forward slash anything. The address is genuine but no contact numbers at all. There is a QR code that opens up a link to a Japanese government bond!? All very dodgy. The envelope was the usual HMRC etc but so little detail elsewhere.

What worries me is that my NI number and initial info re registering for self employment is correct. Does this mean someone in HMRC is using my details? There was no contact number on the letter so I phoned Job Centre Plus who told me that IB was an old type of claim and will get someone to phone me. I’m very unsettled by this though so any previous experience or advice welcomed thank you