Welcome to the MSFN forums!
As a general rule, consulting your browser's Web Console/Browser Console/Error Console will provide useful hints/clues as to the "why" :
... and even if that's "Greek" to you, it'll give prospective helpers a general idea...
The ING Bank site relies heavily on customElements, a web spec (part of the Web Components JS+CSS framework) discussed, by pure coincidence , in posts of this thread's previous page...
CE/WC is a technology originally created by Google (they currently are the only ones who dictate how the web should evolve ), the upstream developers of UXP/Pale Moon, due in part to their aversion of anything Google, put WC support inside UXP in the "back-burner", so to speak; truth be told, there still exist major technical issues to backport/glue-in all of WC to the platform, which, as you might already know, evolved from a now quite "old" Mozilla Firefox forkpoint...
CE/WC are currently behind a disabled pref, in an incomplete/immature developmental state... Third party extension authors try. with various success and targeting selected URLs only, to mitigate lack of CE/WC support in UXP-based browsers ; at this point in time, the extension which holds the best promise for UXP users is one maintained by your compatriot Sebastian Hütter, aka martok, which is called palefill:
https://github.com/martok/palefill/releases
Install the XPI file linked in that page and then kindly ask him to include support for ING Bank, by filing an issue (GitHub account required):
https://github.com/martok/palefill/issues
From my initial testing, implementing support would be as easy as adding in file "./lib/builtin-rules.js" below code:
exports = String.raw`
+www.ing.de
+ std-customElements
+! --
developer.apple.com
std-customElements
For your convenience, I have prepared myself a patched XPI file of palefill,
(.XPI file attachment removed on 202208160022Z)
that you are free to test; works as intended here (under latest St52):
Later addition: Bank sites are a major pain in the posterior for "legacy"/non-mainstream web engines, because Bank IT staff insist on using the very latest Web Specs in them (even ones that haven't yet made it to final state, though this is not the case for CE/WC), as they think it gives the higher members of the Bank's hierarchy (who probably know little about web development), as well as most of the Bank's clientele, a false sense of "on-line security" ...
I'm not saying Bank sites should stick to TLS 1.0/1.1 to merely satisfy old browsers, but not always using the latest Google-derived "shinies" won't make their sites (and their customers' interactions) less secure, would it?