Hello, I have installed Moodle 3.9.1 and when a teacher want to add assignment to his course it gives error. I recreated that error with debug informations. None file was modified and it was working at beggining , but then somehow it stopped working.
In attachment a file with error.
This looks like MDL-68879 (see also this reply).
Try editing lib/dml/mysqli_native_moodle_database.php changing line 1138 from:
} else if (is_float($param)) {
$return .= $param;
} else {to:
} else if (is_float($param)) {
$return .= str_replace(',', '.', $param);
} else {