At the time of this post and answer (September 5, 2025, at approixmately 10:40 UTC-4), it is known to Canonical and many people who are actively working on development, patching, etc. that there was an outage on the primary public repository servers. All security updates and other update pockets were unavailable, and the upload / processing queue for Security updates among others was obscenely backlogged as a result of the outage.
This was reflected on https://status.canonical.com as acknowledged incidents:
Both of these incidents were marked resolved after 36 minutes of outage time at approximately 04:30 UTC-4.
However, it is known that the backlog caused by the outage is causing the mirrors and security updates to be 'broken' at this time due to the backlog in the queue for processing. This queue is currently very large and there is likely to be issues with mirrors at this time during the update process. As such, you need to be patient and try again several hours from now (up to 24 hours from now as well if you want, just to make sure stuff stabilizes first).
There is nothing you can currently do about this except wait it out. Mirrors that synced during the outage time or are mid-sync right now during the large queue backlog may get the 500 errors as well on the mirrors.
Alternatively: consider using a different mirror - https://launchpad.net/ubuntu/+archivemirrors has a list of known mirrors in each country. I'm using AWS's mirrors right now and they work (though I got those from EC2 instances, such as http://us-east-2.ec2.archive.ubuntu.com/ubuntu/ which is explicitly one of the servers on EC2's cloud spaces and seems to work fine.)
It seems this may have been resolved as of September 7th, 2025, but if you run into this specific issue you may still be impacted by this noted issue.