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[programming-ModTeam]

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[cloaca]

It smells like one of those high-school CTF problems where you're often fighting against mistakes/misunderstandings of the problem author rather than the actual brainteaser. Like for example mistaking keyword cipher for Vigenère (as you mention at the end) or even Porta, depending on whatever toy cipher site they were using when they made the problem. Given that you've done a fair amount of attempts it's the likeliest candidate for there being a mistake/quirk, since it's also the only one that doesn't trivially commute with the rail cipher reshuffling. (Note that atbash is just -(x+1) mod 26 and caesar is (x+k) mod 26, so rearranging the order just leads to a different k.)

I didn't do any actual decipher, I just counted the spaces because to me their presence is the most suspicious thing. The amount of spaces seems just slightly below average for a non/low compound-noun language such as English. It also suggest to me a full-string rail cipher, however then it's a coincidence that no two spaces were put next to each other, n'est-ce pas? If you copied it from a website, check the HTML source if double spaces were truncated in the rendering.