Hello Dimitar, one of our users experienced issue this again today. As the other posters have said, the issue pops up rather randomly, making it very hard to get feedback other than "I was copy/pasting". We will continue to try to narrow down what they are doing.

In the mean time though, I thought my analysis of the stack trace points to what seems to be an obvious bad assumption in LayoutElement SetParent. 

Are you going to pass my feedback along to your development team?

Looking at the source code one can see that LayoutElement SetParent does this:

...

...

Note how it does not check newParent for null before dereferencing it, yet looking at ValidateParent, which is called only in debug mode, if I read the isValid expression right one can see that the last valid value is

...

|| newParent == null;

...

so a null newParent seems to be valid? There are code paths that return null when calling LayoutBox GetParent() and eventually DocumentStructureCollection.GetParent(this) and this potential, if unlikely, null result is fed to SetParent.

I suspect this causes the null reference exception that's triggering the infinite loop in our case.

Could the fix be as simple as a null check here if null is expected?

We're experiencing this issue as well, and in our case it causes the infinite loop described here https://feedback.telerik.com/wpf/1461453-pasteoptionspopup-throws-an-unhandled-dispatcher-exception-in-an-infinite-loop. I added a comment in that feedback thread but since it's closed and it eventually leads here I figure I'd post it here as well.

A search yielded this https://stackoverflow.com/questions/14211306/stack-overflow-after-an-exception-in-control-layoutupdated-if-dispatcherunhandle.

It turns out that our application does register a handler for DispatcherUnhandledException in which we display a MessageBox.

The SO post seems to say that displaying the MessageBox triggers another layout pass which then likely throws the initial exception again and so on, in a loop which in our application results in a cascade of MessageBoxes like in the attached screenshot.

We're trying to obtain more information from our user to hopefully create a reproducible scenario.

We have the same issue with 2021.1.223.45. Same StackTrace.

400 users, at least 4 crashes a day with this error.

We can't repro, but users say it may be when they copy/paste from Word/browser/Outlook and/or use backspace button.