Page Edit screen is blocked, undefined index

[photocurio]

I have Simple Cache and memcache legacy active. It works, but not when trying to edit a page.

When trying to edit a page I see a white screen and a loading indicator. If I kill page loading, and re-enter the URL, the edit page screen does load.

Then the debug bar show this notice:
/wordpress/content/plugins/simple-cache/inc/dropins/memcache-object-cache.php:217 - Undefined index: delete wp_set_post_lock, update_post_meta, update_metadata, wp_cache_delete, WP_Object_Cache->delete

I am using Wordpress 5.4 and Simple Cache 1.7.4.

[simongcc]

I think I have encountered similar thing.
Wordpress 5.5, Simple Cache 1.7.4

undefined index encountered with the following line
wp-content/plugins/simple-cache/inc/dropins/memcached-object-cache.php:458

Thanks a lot.