Those strings of 43 characters trailing .well-known/acme-challenge are tokens that can be used for manually creating DNS TXT records that Let’s Encrypt is unable to process itself during DNS validation (the same way it’s unable to process the .well-known/acme-challenge directory during http validation). Sometimes a manual TXT record workaround gets results when all else fails.
Under ideal conditions manually adding tokenized TXT records shouldn’t be necessary. Based on LE’s message “Timeout during connect (likely firewall problem)” there could be some other network obstacle, like missing glue for nameservers that causes delayed lookups.
If you want to try it, a TXT record needs two pieces of info:
- Record name: _acme-challenge.domain.tld
- Text record/value: xxxx-43-character-token-string-of-text-xxxx
Use the most recent token LE displays in an error or log, not just any token that could have expired.
If you’re agreeable to a wildcard certificate a single TXT record for the parent domain should be all that’s needed.