Help some users like #16832#1851
There are many users reporting similar problem: if the SECRET_KEY
mismatches, some operations (like 2FA login) only reports unclear 500
error and unclear "base64 decode error" log （some maintainers ever spent
a lot of time on debugging such problem)
The SECRET_KEY was not well-designed and it is also a kind of technical
debt. Since it couldn't be fixed easily, it's good to add clearer error
messages, then at least users could know what the real problem is.
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Co-authored-by: delvh <dev.lh@web.de>
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