Can it be that the content language for en-GB is missing? I don’t mean the language packs or extensions, I mean the content languages, that’s an extra point in the manage area of the system panel. That’s where the sef property should come from which is mentioned in the error message.

Hello Richard, you are right: the en-GB was unpublished. After publishing it, the error disappeared.
Is it an idea that a hint is displayed instead many error messages in case this happens?
Thanks for your help.
Regards

Is it an idea that a hint is displayed instead many error messages in case this happens? Thanks for your help. Regards

@peterhulst Well, I would indeed expect a useful alert instead of an uncaught exception when you enable the language filter and the content language is not published.

I think that's a bug.

On a multilingual site there is also a module shown in the administrator (in the default Atum template it's shown at the top) called "Multilingual Status" or so. That opens a modal which shows what's missing on a multilingual site, and it should show that the content language is not published.

I've added a sentence about the unpublished content languages to the description (initial post) of this issue so quick readers can better see what the issue is.

Thank you for the comment.

When you unpublish all the content languages you already get a nice error message

Thank you Brian!
Indeed this message is very clear.
Regards,
Peter

Well that's the nice error message when you unpublish the site languages. But when you have them unpublished and enable the language filter, we have the PHP warnings.

So should we leave this issue open and hope someone fixes that, too? Or shall we close it as expected behaviour?

I created a PR. Please test #43232. Since we have a PR, I'm closing this issue.