Follow up: Safepay does not handle in particular a website (except a handful with known troubles, most of them regarding user agent). It is, possible, however - and I saw it for multiple websites - that they "expect" certain version of browsers, and if these are not how the website expects, it sometimes rejects the login (if this is the case).
There are websites that displays explicitly "Your browser is too old" - Safepay is build based on Chromium, but we don't release at the same pace as Chrome - but most of such websites are simply displaying "it didn't work".

I just "tried" to log using some dummy credentials, and it redirected me to a page stating
"Your account cannot be currently accessed.
Outdated browsers can expose your computer to security risks. To get the best experience of Discover.com, you may need to update your browser to the latest version and try again."

The currently release Chrome version is 133, while the current Safepay is based on 130. On my experience, there are websites which accept maximum two versions behind (so they accept 133+132, sometimes 133+132+131) and it seems Discover.com applies this. While this is understandable from a liability point of view (nobody wants users attempting to connect to the website with a 10-year old browser), but it is also a little bit too eager to be "du jour" and hampering software which is being stable but releasing at a slower pace.

The good news is that I am exactly in the middle of developing the update of Safepay to be based on 133 version, which I hope to have it released in the next weeks an a subsequent update will fix this.