Investigation has been completed.
It’s not DOS related. I can rule that out, because I have other other onions with non-pubic onion domain names. (Such as for fetching e-mail over onion.)
The only way to fix a broken onion is the following cumbersome procedure.
Tor / Tor network issues / bugs.
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stop tor@name.service on the server
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manually delete Tor consensus and state files on the server
/var/lib/tor-instances/name/cached-certs
/var/lib/tor-instances/name/cached-microdesc-consensus
/var/lib/tor-instances/name/cached-microdescs.new
/var/lib/tor-instances/name/state
(Not all files may need deletion. Perhaps “only” deleting the state file would be sufficient.)
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restart tor@name.service on the server
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restart tor@default on local system
These were the only steps requires to temporarily “fix” this. (I wouldn’t call this a “fix”, not even a workaround.)
This is not a safe procedure and should not be required. Not something a “real” onion host requiring location privacy can afford. (Not a safe procedure because it replaces Tor entry guards.)
It’s also not a server configuration issue, because I have multiple servers with identical configuration except for the onion service domain names and private keys. So no other action was required to fix this other than the cumber some procedure.