Curious I am? You don't give your website address.
But is your site in the root of your host and is that the [html] directory or do you have a long path inside such as [public_html/data/web/e65807/html/blogs/] where your site runs from.
The reason I ask is that the error is in a directory a few levels above your [blogs] site

the website: https://fokus.genba.org (the post is not published yet)
I got a blank page with the exception of the the error message, quoted above.
Saving the page "update to now" makes no problem. It happened when setting to a date in future.

I run my differential diagnosis routine and conclude, that this fatal error may be related to "webmention". In case a post is published by "set to now" there is no problem. The problem results in postponing a saved public post to a later date. May be that webmention cannot handle a publicly saved post and a postponed release.

I tested this also with different skins. Result: It is not a skin that causes this bug.

Ok I tend to use older stuff on the webs so webmention is not on my list.

I'm imagining that b2evo has the option for you so send a webmention.

But it seems that as it is designed to send a message to an original author that their post/page/ etc. has been linked to it makes sense that you cannot add a webmention event to a future post.

Saying that b2evo could understand the problem and delay sending the webmention until the future post date becomes active.

I hope that is some understanding of the issue and problem.

It is very likely indeed that the issue is related to setting a date in the future. We'll fix this ASAP.

Hello @saunders, fixed in commit https://github.com/b2evolution/b2evolution/commit/f4f400b92cf630c9c583cd444e500bc1d5d60a2e.

@fplanque @yurabakhtin thank you for the fix! Works!