@marleen-meert,
-First disable the Headers and Footers plugin. If you don’t have access to your WP site anymore because of the Error, you go to the Plugins folder on your hosting (WP content > plugins) and rename the plugin Elementor Headers & Footers. This should give you your access to WP back.
-Next, install the plugin WP Rollback.
-Next, make sure the Headers and Footers plugin is deactivated. Probably it is.
-Rename the folder of the plugin to its original name.
-Refresh the plugin page in WP
-Click on Rollback on the Headers and Footers plugin, choose earlier version. And Roll back.
-Activate Headers and Footers plugin. But DO NOT UPDATE. I tried; it will crash your site again!
Hope this helps you out for now.