thank you!
Turned out that this was a conflict with ‘Table of Contents Plus’-plugin (https://da.ww.wp.xz.cn/plugins/table-of-contents-plus/).
During the debugging I also realized that this plugin haven’t been updated for the past 2 years, so will find a replacement.
Again, thank you for your support. This was really helpful!
After doing it for two days it now stopped on both domains, without me changing anything.
