In the version 2.3.5 there was a changeover of the PHP libraries for the MySQL-access. To ensure the compatibility of MLF2 with PHP7 we changed the PHP-library for the MySQL-access from mysql to mysqli (please note the trailig "i") in this version. That causes the fail of an direct update from a version below 2.3.5 to a version higher than 2.3.5. Please read the notes at the beginning of the update-guide before you begin such an update.
Trenne niemals Müll, denn er hat nur eine Silbe!