Follow-up: buttons working again by adjusting CSS z-index (I use a custom layout in a self made theme: the plus-button is at the top left of the quantity box, the minus is at the bottom left). So always check functionality when updating a plugin.
The PHP-notices still remain to be fixed. Also occurring with standard WordPress theme Twenty Nineteen.
Thanks a lot Rui.
The new version fixed it. Can I continue to use this or should I wait until the official release?
FYI: 2.9.1 is also working fine. Only 2.9.2 had the problem.
