Is this blocking?

5.X have same issue ,
Only stat show error (500 error)

500 : Erreur interne au serveur
La propriété « Response_618224.618224X900X7852SQ001 » est indéfinie.

Is this blocking?

Clearly … have the information about a broken survey data 5 month after is totally blocking !
With 2.6 and 2.50 : you get the email at minimum : you know there are an issue.

partial_block because it does not happen to everybody ?

I am not saying is not important. Seems like it is.
I am saying it doesn't sound like an issue with "blocking" category.

I believe we could implement an email notification in case of 500 erros (and others).
Yii don't have something about it?
Isn't that a logging type?

partial_block seems OK.
Priority: Important

I believe we could implement an email notification in case of 500 erros (and others).

It's not a 500 error here …

in 2.6 : if such error happen : admin user receive an email,
Not in 3.X and 5.X

https://github.com/LimeSurvey/LimeSurvey/blob/59ad72dbc5025da84a015b3fef11797de666d6db/application/helpers/frontend_helper.php#L569 didn't happen

It's not related to Yii error or anything else …

And about show warning in browse it's really a think to do : "Your database seems to have an issue, some question didn't have related columns" or somthink like that.

We surely use an empty() but must use an isset ?

Maybe we should add here https://github.com/LimeSurvey/LimeSurvey/blob/master/application/helpers/expressions/em_manager_helper.php#L5065

Something like this (which is used already on the code)

                    LimeExpressionManager::addFrontendFlashMessage('error', $message, $this->sid);
                    return;

I would add it to DEV as to have more testing time.