I just found another thread about the calendar and the relevant error message and have deleted the calendar function from modules list and there is now no error message. I hope I don't come to regret this! I never use the calendar. I think I might have been able to hide it in some way but took the nuclear option.
https://moodle.org/mod/forum/discuss.php?d=398537
https://moodle.org/mod/forum/discuss.php?d=398537