I had the same issue. Setting compatMode.firefox to true also worked for me. Here are the errors I got through the new calendar creation process.
Lightning:[calGoogleSessionManager] Creating session
mye...@myprovider.com.au gdataSession.jsm:73
Lightning:[calGoogleSession] Token expired 1574339162 seconds ago, resetting gdataSession.jsm:341
Lightning:[calGoogleCalendar] Logging in session
mye...@myprovider.com.au gdataSession.jsm:237
Lightning:[calGoogleCalendar] No access token for
mye...@myprovider.com.au, refreshing token gdataSession.jsm:302
Lightning:"[calGoogleSession] Adding item
https://www.googleapis.com/tasks/v1/users/@me/lists to queue" gdataSession.jsm:358
Lightning:"[calGoogleSession] Adding item
https://www.googleapis.com/calendar/v3/users/me/calendarList to queue" gdataSession.jsm:358
DEPRECATION WARNING: onPromptStart has been replaced by onPromptStartAsync
You may find more details about this deprecation at:
https://bugzilla.mozilla.org/show_bug.cgi?id=1176399jar:file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/omni.ja!/components/msgAsyncPrompter.js 27 _promiseAuthPrompt/<
jar:file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/omni.ja!/components/msgAsyncPrompter.js 21 _promiseAuthPrompt
jar:file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/omni.ja!/components/msgAsyncPrompter.js 44 runnablePrompter.prototype.run<
resource://gre/modules/Task.jsm 326 TaskImpl_run
resource://calendar/modules/utils/calWindowUtils.jsm 26 openCalendarWizard
chrome://calendar/content/calendar-common-sets.js 358 doCommand
chrome://global/content/globalOverlay.js 84 goDoCommand
chrome://messenger/content/messenger.xul 1 oncommand
Deprecated.jsm:77
Lightning:[calGoogleCalendar] Failed to acquire a new OAuth token for
mye...@myprovider.com.au data: {
"error": "invalid_grant",
"error_description": "Malformed auth code."
} gdataSession.jsm:247
Lightning:[calGoogleSession] Authentication failure: {
"error": "invalid_grant",
"error_description": "Malformed auth code."
} gdataSession.jsm:272
[Exception... "invalid_grant" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: resource://gdata-provider/modules/gdataSession.jsm :: login/authFailed< :: line 274" data: no] (unknown)
Regards,
John.