My wife just tells me that for a couple days, DIA wasn't working for anyone, until the 6th (she didn't know when it started).
We just went on a holiday to the EU, and it was a struggle to get DIA to work for the covid cert. I have a Pivdennyi account from a couple of years ago, still active, and I can use it to log in to DIA, but it didn't work on creating an electronic signature for the covid QR code.
So, I went and opened a Privatbanka account, and to create a DIA account, needed a different email address and phone number than the one I used with the login from Pivdennyi (basically, new login credentials). That's when I noticed that Pivdennyi bank had used my ID card number as my tax ID number so it didn't match up. But the Privatbank account DID use my UA tax ID number (we got to sign up for a family doctor in the health care system) and I was able to create an electronic signature via the DIA website, which I used to generate a QR code. But the app still doesn't work, however, I took a photo of the QR code on the monitor and it "reads" on scanner devices successfully.
Not sure if this helps with your situation, just make sure your bank has your correct UA tax ID number in their systems, that was the core problem on my first account.