I have a working adaptable theme (copied my setting across manually) in a test Moodle (but it had no copy data in it), and yesterday was able to fix the calendar block issue. The calendar was not available from the drop down menu to start with so by setting all the dashboard builder blocks to setting 1 - the calendar then appeared in the drop down menu. I added the calendar and the error message went.
So I then transferred the test theme over to another Moodle with a copy of our data in it, but experienced the same 'Invalid' error message, meaning that the issue is hard coded into the database. So even if I add a new downloaded adaptable theme from Moodle.org on top of our real data Moodle the issue will still appear.
I was upgrading from Moodle 3.1 - to Moodle 3.5 using adaptable theme.
As I have now spent to long on this issue I have decided the best action is to delay the update and use a Boost or Fordson theme. I will be building the new theme in a Moodle with a copy of the data files in it, due to what I said above.
Good Luck