SharePoint/Excel - This content presents a potential security issue. Do you trust this content?

Yes, this is a known behavior and it usually starts right after an Office update or a change on the Microsoft 365 side, which matches the "since this morning" timing.

The prompt appears when Excel no longer treats your SharePoint location as trusted. Try this first: open Internet Options, go to the Security tab, click Trusted Sites, then Sites, and add your tenant's SharePoint address there. Restart Excel after that.

Also check how the files are being opened. If some users open via a mapped drive, a network path, or a different address form (non-secure instead of secure, or the short tenant name), Excel sees it as untrusted even though it is the same site. Make sure everyone opens through the same secure address.

If you are an admin, you can push the fix with group policy by adding the SharePoint address as a trusted location under the Microsoft 365 Apps trust center settings.

If it still happens, post back with the exact Excel build numbers from File, Account, and whether it happens on all files or only ones with external links or macros.