Hi Gijs,
Please take my sincere apology for not being there.
I have just checked and there are Joomla and Helix Ultimate updates available. It would be better if you update all of them.
However, if you remove the tabs addon from here:
https://prnt.sc/87tsvNmLAGLD
Then the page opens with frontend editor. It seems tab addon has conflicts with something.
Please update your Joomla and others if possible and let me know. Here is supporting blog post:
https://www.joomshaper.com/blog/how-to-update-your-site-to-joomla-4
Keeping full site backup is always important.
Best Regards