"There are no date fields in this database" after adding a date field

I'm trying to switch from Event to Date+Calendar.
I have added a Date field to the Event content type.
A message said that this field had been successfully added and I can indeed see it on the "Manage fields" screen.
However, when I try to use date_tools' import, it says that there is no date field in this database.
I made sure my date field has an optional end date and the timezone is set to date as recommended.
Any help greatly appreciated.
Thanks !