In the file db/install.php line 11-12, the fields intro and introformat are defined with NOTNULL="true" while in db/upgrade.php line 1826-1836 they are defined to accept null value. The result is that on a fresh install null values are not accepted while on a plugin upgrade they are. I've had data backuped with null intro on a Moodle with upgraded game that couln't be restored on a Moodle with a freshly installed game.