I had to remove hide-my-wp after 7 years! It makes the WP backend empty - only plugin page is visible but does not work properly.
I had to remove it via FTP because in the WP backend it was impossible.
After removing the plugin and updating everything again I tried a fresh install of Hide-my-wp but at activation of the plugin the WP backend pages (not the sidebar) went blank again... (Hide-My-WP wasn't even set-up/configured yet)
So I'm waiting for a new update