I followed the old discussion as a lurker, but i might have a solution to this problem, which i run into myself the last couple of weeks.
To me it looks like postbank.de doesn't resolve, only if Cloudflare is one of the chosen upstream-providers. After i disabled 1.1.1.1 an swiched to Google & Level3, postbank.de has no problem loading.
On top, i found a just 18 days old support-posting from a Cloudflare Staffer, explaining that they have trouble resolving postbank.de
The root cause is that postbank.de nameservers return slightly invalid proof of non-existence, which breaks resolvers implementing RFC8198 . We’re trying to work with the operator to fix this, as the only workaround is disabling DNSSEC which unfortunately brings its own problems.
@don_misu : You might want to consider removing the "solved"-Checkmark on top of the old discussion, pointing to whitelisting the scam-domain "[...]-postbank.ru" - as this does not work, it might cause more harm than good.