Hi Thimo,
thanks for you fast reply. It comes right after clicking the rebuild cache.
I dont know about a firewall, I will check it on siteground. Every catch option is of thats for sure.
Also wordfence I deleted.
I have a JSON generator on my site that I have to use often to fix things, but its not working for me now. For now I use the Javascript code, that works fine for now.
If you have any other ideas, please let me know. I keep you posted when it works with the plugin.
Have a good one.
Bob