I found the problem. The culprit is the string $string['update'] in lang/<language>/moodle.php. If this string was adapted for a language, say french, then the problem occurs. If this same string is not adapted (i.e. is not present in the language file), then the problem doesn't occur.
Furthermore, if I change the same string from the french $string['update'] = 'Modifier'; to the original english $string['update'] = 'Update'; in my language pack, the problem disappears again.