The container must have reachable as it is running next to the Lnbits container, and I tried the curl command literally a few secs later, successfully.
That said, I went through the process again just now - and it magically works now. I can save the incoice key, without error message. Weird. but at least it works
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lnbits.mydomain.tld
) but LNbits returned this error. Maybe it wasn't able to reach the container when it tried but it would work on retries?