It looks like one or more of the regr.json files in /etc/letsencrypt/accounts/ has gotten corrupted.
The account directory has one directory per ACME server (e.g. /etc/letsencrypt/accounts/acme-v02.api.letsencrypt.org/ for the Let's Encrypt production ACME server) followed by a directory per "directory" endpoint (e.g. /etc/letsencrypt/accounts/acme-v02.api.letsencrypt.org/directoryfollowed by a hash of the account of that ACME endpoint. There you'll find the JSON files for your account, includingregr.json`.
regr.json should be valid JSON and contain a body (which seems to be empty for me) and an uri field, containing the account URI for that account.