Design errors are a class of mistakes. Nobody can help you with design errors. Functional programming, immutability, static typing et al address other kinds of mistakes.
> when in fact most of the serious information leakages do not originate from such problems
We've already established that the parent post made a mistake in thinking FP is about safety against attackers / information leakage. Why are you still talking about this?
> because those languages are so insufferable that they induce design errors
[citation needed]
What's your actual experience with these languages?
Same goes for the proponents of formal stuff like Coq. Even worse, because those languages are so insufferable that they induce design errors, the number of which will grow each time a poor soul will have to maintain the... thing. Then, all right, there is no mistake, the result maps perfectly to all your design errors.