Technical info:

Add frontent validation.

If front end validation fails then in

application\models\services\QuestionAggregateService\SubQuestionsService.php

the exception here:

    if (!$subquestion->save()) {
        throw new PersistErrorException('Could not save subquestion');
    }

should also give back getErrors() from the model.
That should also be handed back in the ajax reply.

@DenisChenu @gabrieljenik who can take this one?

Partial fix : https://github.com/LimeSurvey/LimeSurvey/pull/3729

As long as I don't understand the purpose of the services and what they're supposed to return: I touch them as little as possible.

Yeah, I will take this for now.
There seems to be the same problem with answer options

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=36068

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=36067

Yeah, I will take this for now.

My current issue is more on application\models\services\QuestionAggregateService\SubQuestionsService.php

And the first question is Why there are no translation of error ? What purpose ?

The fix is different :

1. use SubQuestionsService with try/catch
2. in SubQuestionsService add a getLastErrorForDisplay
3. In cacth use getLastErrorForDisplay

or : remove usage of SubQuestionsService and use model directcly …

There seems to be the same problem with answer options

Can be done with html attribute : i can do it.

Already did it: https://github.com/LimeSurvey/LimeSurvey/commit/bccddcfc05cd4e852522ae7eed342ab241e514a5 - all good ;)

Fixed in Release 6.4.7+240219