Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
I couldn’t find an article or answered topic that says what causes an intermittent openresty 502 on a static page, or how to get a verifiable certificate chain on an existing account. So I can’t tell you which of the three causes you list is involved. I also didn’t find the documentation passage you mention about CA-chain installation.
One check would help separate the two problems. Does the 502 also happen on the free subdomain on the same account (the one under free.nf), or only on baukompetenzzentrum.org? If it only happens on the custom domain, that points toward the domain or its certificate. If it happens on both, it’s more likely not specific to your HTTPS setup.
It would also help to know what a normal desktop browser shows when you open the HTTPS site. Does it show a certificate warning, or does the page just load or 502? That tells us whether the “unable to get local issuer certificate” message comes from those checking tools or from real browsers too.
Another recent topic describes the same openresty 502 symptom on a different site, but it has no answer yet, so I can’t say whether the cause is the same:
Your account and files look fine from your checks. If the 502 shows up on both domains, it probably needs a person on the InfinityFree side to look at it, because nothing you can change in the account would affect it.