Anyone knows about the error when logging in to CMS?

No but problems like this are often because of problems with your user's assigned role, or with permissions on script deployments.

Sometimes you can find out the reason for the error by checking Network and Console tabs in your javascript console of your browser, or the script execution logs,

I'm having this same error happen in the same place with an "out of the box" (unmodified) version of Kilimanjaro Extension Cart/Checkout. It just started today, and I haven't made any changes to code or settings. But it's also the night before my 2024.2 version update.... lots of unannounced back-end changes seem to be happening with this version rollout. Whatever is causing the unexpected error message isn't actually preventing login or checkout. But it's making my customer nervous and question if they want to provide their info (aka, lost sales). I inspected the error and it appears to be coming from checkout.js with the browser reporting "*Impossible to parse backend error*"