Possibly caused by PR #40462 ?

Update: I don't think it was that PR, but maybe it unmasks the problem somehow? When debugging, I saw that the right side of the side by side view should use the edit layout but tries to use default layout.

I have that error on the edit view of the article at the right side of the associations side-by-side view. There is no target language selected. As soon as I select a target language, the error disappears and the article edit view at the right side is ok. When I de-select the target language after that so none is selected, the article edit at the right side is empty like it should be, and the error still doesn't happen again.

So it seems to be a problem with the initial value of that target language somewhere.

Update: It happens only when the edited article has an association. For unassociated articles the problem doesn't happen.

Next issue: The "Clear" button at the right side of the side-by-side view doesn't do anything, regardless if I have used the "Change Target" button before or not.

Next idea for a possible cause: PR #40462 . Ah, it's the same PR again which I already found before. Just checked another place in the code this time.

The network analysis in my browser's developer tools show an internal server error for the GET request to this URL: https://www.joomla-50-dev.vmubu01.vmnet2.local/administrator/index.php?option=com_content&view=article&extension=com_content&tmpl=component&task=article.article-en-gb&id=1

It seems the &task=article.article-en-gb is either the problem or a symptom.

When selecting the right target language, the GET request goes to
https://www.joomla-50-dev.vmubu01.vmnet2.local/administrator/index.php?option=com_content&view=article&extension=com_content&tmpl=component&task=article.edit&id=1

The same can also be found for the src attribute of the iframe which containts the article at the right side.

@Fedik are you willing to work on this one or should I give it a go?

I have found where it comes from,
Please test #42096