2017-09-27 Enigmail 'partial decrypt'
The imap server where I fetch my work mail changed and suddenly the enigmail plugin on one system could not decrypt gpg-encrypted messages anymore with a 'partial decrypt' error. I remember seeing this before but had to look up the details. Found again, it's a setting in enigmail (not in the general thunderbird preferences!), Enigmail → Preferences → select Advanced → Only download attachments when opened (IMAP only) has to be unchecked. Answer found via Resolving Thunderbird/Enigmail decryption errors for encrypted emails with large attachments