Hi threadi,
thanks for jumping in. This here would be the plugin. I have resolved the situation by changing the date & time of the system, so that this fake subscription renewal does not get triggered automatically a couple of seconds after a restore, thereby causing this fatal error. I then have restored a new version, wherey I deleted the plugin from the backup before restoring. And it worked. God, I am so glad..
However, the Elementor issue was not related to this but – as you correcly suggested – to an imcompatibility issue with a plugin called “WBCom Design LearnDash Dashboard”. I was basically using the User role Editor to grant 3 roles to myself ( Group Lead, instructor role from this plugin and shop manager role from WooCommerce). This somehow disactivates Elementor. No clue why, but this is also resolved.
Many thanks for your valuable insights!