Notification email not send

Since a couple of weeks the notification email that the backup has been informed is not being sent for one of my sites, although the settings asking this has not been changed. I have another site, at the same hoster, where the notification mails are sent. Same settings, same hoster. The last lines of log of the site not sending emails are:

0219.318 (0) Sending email (‘Bestanden en database (Volledige backup)’) report (attachments: 0, size: 0 KB) to: masbe…
0219.331 (0) PHP event: code E_WARNING: json_decode() expects parameter 1 to be string, array given (line 177, wp-content/plugins/updraftplus/includes/Google/Auth/OAuth2.php)
0219.333 (0) Exception occurred when sending mail (Google_Auth_Exception): Could not json decode the token
0219.335 (0) De backup is zo te zien geslaagd, en is nu afgerond.

The lines 331 and 333 seem to indicate something, but not understandable for me.

Any idea ?

Thanks,
John