TB78.1.1, profile upgraded from TB68.11.0
OpenPGP is enabled.

Attempting to import a private key into OpenPGP from a backup file. Upon selecting the file, TB recognizes that this is a private key, and then presents the passphrase prompt.
Passphrase is being entered, then nothing happens anymore, and the import dialog kind of hangs.
This is shown in the error console:
CryptoAPI.sync() failed result: Error: rnp_key_protect failed
importKeyBlockImpl chrome://openpgp/content/modules/RNP.jsm:1676
sync chrome://openpgp/content/modules/cryptoAPI/interface.js:51
importKeyFromFile chrome://openpgp/content/modules/keyRing.jsm:361
openPgpImportStart chrome://openpgp/content/ui/keyWizard.js:761
wizardNextStep chrome://openpgp/content/ui/keyWizard.js:194
wizardContinue chrome://openpgp/content/ui/keyWizard.js:143
_fireButtonEvent chrome://global/content/elements/dialog.js:487
_doButtonCommand chrome://global/content/elements/dialog.js:466
_handleButtonCommand chrome://global/content/elements/dialog.js:460
interface.js:46:17

Uncaught (in promise) TypeError: res.importedKeys is undefined
importKeyFromFile chrome://openpgp/content/modules/keyRing.jsm:373
openPgpImportStart chrome://openpgp/content/ui/keyWizard.js:761
wizardNextStep chrome://openpgp/content/ui/keyWizard.js:194
wizardContinue chrome://openpgp/content/ui/keyWizard.js:143
_fireButtonEvent chrome://global/content/elements/dialog.js:487
_doButtonCommand chrome://global/content/elements/dialog.js:466
_handleButtonCommand chrome://global/content/elements/dialog.js:460
keyRing.jsm:373:36

Upon closing the tab, or restarting Thunderbird, the key has not been imported.
The very same key has always worked in betas.

This may be related.
After starting Thunderbird, this shows up in the error console:
core.jsm: startup.continueStartup: error Error, secring.gpg exists, but cannot obtain password from encrypted-openpgp-passphrase.txt
ensureMasterPassword@chrome://openpgp/content/modules/masterpass.jsm:58:13
continueStartup@chrome://openpgp/content/modules/core.jsm:136:32
startup@chrome://openpgp/content/modules/core.jsm:158:20
getService@chrome://openpgp/content/modules/core.jsm:472:18
getService@chrome://openpgp/content/modules/core.jsm:210:31
init@chrome://openpgp/content/BondOpenPGP.jsm:78:23
@chrome://openpgp/content/BondOpenPGP.jsm:97:13

The master password has been entered upon Thunderbird start-up.

Seems to be related to Bug 1656287

(In reply to Andreas from comment #2)

Seems to be related to Bug 1656287

Thanks for that. I did follow the suggestion in comment #20 of bug 1656287. Deleted the file secring.gpg, and then successfully re-imported the private key. This fixed the problem.