menu permanently expanded/unfolded

Hi,

Since installing WP 5.5.1 last week, the menu’s on the left hand side of my website are permanently expanded, i.e. they display all subpages underlying main pages. Before, users were required to click on a main page in order for it to expand.

To have the menu’s automatically expanded like this is undesired for my website because it contains several hundreds of pages.

Can anybody advise on how to revert back to the old situation? Your help is much appreciated.

Regards,
Rutger

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