Using the Mailbox Migration on SmarterMail v16.2.6442 Enterprise edition to import GMail, Contacts, Calendars and Tasks. Email imports fine but no Contacts, Calendars or Tasks are imported. Same error(s) occurs if you select just those items individually.
Calendar
[2017.09.13] 17:31:15 (Gmail) [smuser@domain.tld] - Fetching calendar items
[2017.09.13] 17:31:16 (Gmail) [smuser@domain.tld] - [imap.gmail.com:user@gmail.com] Exception: Error:"unauthorized_client", Description:"", Uri:""
[2017.09.13] at Google.Apis.Requests.ClientServiceRequest`1.Execute()
[2017.09.13] at RelayServer.MessageRetrieval.Gmail.CalendarRetreiver.Fetch()
[2017.09.13] at SmarterTools.SmarterMail.RelayServer.MessageRetrieval.GmailRetrievalSession.#GGb(User #kdb, GmailAccountConfig #l6i)
Tasks
[2017.09.13] 23:35:48 (Gmail) [smuser@domain.tld] - [imap.gmail.com:user@gmail.com] Processing started
[2017.09.13] 23:35:48 (Gmail) [smuser@domain.tld] - Fetching task items
[2017.09.13] 23:35:49 (Gmail) [smuser@domain.tld] - There was an error executing the list request on Google's API.
[2017.09.13] 23:35:49 (Gmail) [smuser@domain.tld - [imap.gmail.com:user@gmail.com] Processing completed
Contacts
"It looks like your Google account is already authenticated." pop-up error. No detailed error msg in logs.
Had this error occur on 3 different Google accounts, one was plain-jane GMail, one was Google Apps Legacy, and one was a premium GSuite account. The setting "Allow Less Secure Apps" was toggled for all accounts.