This support ticket is created vor 5 Jahre, 3 Monate. There's a good chance that you are reading advice that it now obsolete.
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I did some research and WordPress offers some relatively newer functions, which can be used to first get the ID of image attachment and then it's caption text:
I've broken it down into smaller chunks and the shortcodes inside are now working from that template too. I'm sorry, I should've spotted that issue earlier and this seems to be the reason why I was not able to reproduce this on my own test website.