Can you see any errors in the F12 browser console?

Not sure what you call F12, but in the Vivaldi Console I see the screenshot.

What is show appear when the page load, but when I type e-mail or click button nothing appear.

Console.png (71,856 bytes)   

Console.png (71,856 bytes)   

OK, thanks, I think that's just warnings.

Yes but no information helping us to understand why the problem happen.

Will try to reproduce

Confirmed.

It can not be fixed using input type="email" multiple directly ?

Sorry, applied a status change to the wrong ticket

This is a minimalistic approch for the fix.
Seems some general code from LTS related to validations and button disabling is not on master.
A more general approach can be taken, but imapct would be very wide.

https://github.com/LimeSurvey/LimeSurvey/pull/2748

@ollehar, maybe we can review that for the develop branch. Actually, we have something already close to finish which could be good for develop
https://github.com/LimeSurvey/LimeSurvey/tree/bug/18449--Impossible-to-add-participant-after-correcting-an-e-mail-address-error

maybe we can review that for the develop branch.

We really need to use HTML5 input validation

Extra button must trigger real button inside the form, and nothing else.

Tested OK

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=34148

Fixed in Release 5.5.0+221219