Cannot move menu item without getting error

I’ve just created a new page and added it to the menu as normal, but unlike normal I have experienced an error. It’s okay when the menu item is introduced at the bottom of the list (i.e. a new “main” item), but as soon as I try to move it anywhere else, I get an error. On screen after the creation of the menu, near the top of the screen, I get “ey 0 in?/var/www/vhosts/mysite.org/httpdocs/wp-content/plugins/if-menu/if-menu.php?on line?310“. I can’t read what comes before the “ey” because it’s hidden by the black WP dashboard menu and won’t let me copy it.

At the same time, if I then look at the site, my home page is full of error messages:

Trying to access array offset on value of type null in/var/www/vhosts/mysite.org/httpdocs/wp-content/plugins/if-menu/if-menu.phpon line111 Warning

The same message appears twice more but showing line 115 on those occasions.

The horizontal menu across the top of the screen has been replaced by a vertical menu on the right. I note the new item is in the right place in the list.

The only way I can save this is to remove the page from the menu.

I have version 0.18.0 (basic) running on Mesmerize Theme (not Pro) Version: 1.6.125, WordPress version 6.5.

Any ideas warmly welcomed…

Thank you