Thanks for all the info and the help. I finaly figured it out.
For the next one, the issue was the plugin ‘Pojo’ (Web Accessibility – WCAG Scanning, Guided Fixes, Usability Widget) this adds alot of HTM code to the ical link, and therefore the link can not be used.
I’ve deactivared the plugin, placed 2 test bookings through our WordPress and now the ical links works and AirBnB accept them.
So for the next one, this is how i can to figure it out:
place the ical link in the browser, a file should start downloading.
Open the file with note pad and copy the tekst in any AI.
Explain your situation and he’ll find the plugin that causes the issue.