Hi Georg,
I’m sorry that you’re running into this issue!
Can you please send me a screenshot of where you’re seeing this error? Is this something that happened following an update? If you haven’t done so already, can you try running through these steps (with WP_DEBUG enabled) to see if you can find any conflicts with your theme or another plugin? If you don’t want to do this on your live site, the free WP Staging plugin will let you create a quick copy of your live site that you can use for testing.
Regarding the slow loading time, I would recommend taking a look at this article, which has some good tips on improving site performance. You may also want to install the Query Monitor plugin, which will give you information on your site (such as slow queries) that will be helpful in narrowing down the source of the issue.
Please let me know what you find!
Thanks,
Jennifer