Last WP version update not supported (yet)?

Hello.
Please be sure to use the latest update of Booking Calendar 8.8.1

Otherwise Probably, its because of conflict with some other plugin or actual theme.
Please try to deactivate one by one all your active plugins at the WordPress > Plugins menu and make testing after each plugin deactivation.
If its not help, please deactivate all your plugins and active the default WordPress theme. And then retest it again.
We just need to find the reason of this issue.

I have tried that, deactivated all the plugins one by one, and all the plugins cummulatively. Also activating the default WP theme didn’t work.

It’s not only the bookings which are invisible: selecting a date doesn’t work either. In other words, the plugin only shows a calendar now and isn’t interactive.

Hello.
I have rechecked your website, and it’s seems that you are using some cache plugin, or some cache system at your server.
For example loading of one of CSS files is looking like this:
https://inhetpakhuis.nu/wp-content/plugins/booking/assets,_libs,_bootstrap,_css,_bootstrap.css,qver==3.3.5.1+assets,_libs,_bootstrap,_css,_bootstrap-theme.css,qver==3.3.5.1+css,_client.css,qver==8.8.1+css,_calendar.css,qver==8.8.1+core,_timeline,_v2,_css,_timeline_v2.css,qver==8.8.1+core,_timeline,_v2,_css,_timeline_skin_v2.css,qver==8.8.1.pagespeed.cc.MKv-mxbIdC.css

This file pagespeed.cc.MKv-mxbIdC.css is not from the original Booking Calendar version, it’s caching file.

Please recheck for any cache plugins, like “WP Super Cache”, “WP Fastest Cache” or “W3 Total Cache”. If you are using someone, please deactivate it or add the exception to the page with booking form for do not cache this page(s).

Otherwise contact support of your theme, and ask them to stop the cache system or add the exception of not caching pages with booking forms and calendars.
Thank you.

I have disabled all caches on my site, without any results… I used another browser to exclude possible browser cache issues, but that didn’t help either.

Your website is still caching.
Please contact support of your server (hosting provider), and ask them to stop the cache system or add the exception of not caching pages with booking forms and calendars.
Thank you.

Ok, some progress here: I found out that deselecting “Use bootstrap CSS on form” restores the functionality of the booking calender, but erases all of the styling… I assure you, no other cache plugins are currently used or activated.

Hello.
I was made the test booking on 26 of April and after reloading of this page https://inhetpakhuis.nu/beschikbaarheid/,
I can see 26 of April 2021 as unavailable date in your calendar.
Kind Regards.

Yes, the functionality of the plugin is back now, but the styling is gone…

Hello.
It’s seems fine for me, relative calendar skin.
Kind Regards.

Yes, it’s ok now, I have changed the skin. Did you change anything on your side? Any idea why it works now?

Hello.
No, I have not changed anything.
Kind Regards.