Please help us help you and fill where relevant:
Your LimeSurvey version: 6.3.1+231023
Own server or LimeSurvey hosting: own server
Survey theme/template: Fruity TwentyThree
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If I send the survey URL via WhatsApp and the participant clicks on it, he/she gets an error message saying that the connection was refused by the server. However, if I send the same URL via e-mail to the same person, on the same mobile, no problems occur, so it shouldn't be a real server problem.
Moreover, this problem seems to be limited to Android, while on iOS the URL works also via WhatsApp.
I just can't figure out what's the issue...
Thank you!
Cristiana

is this an issue with one specific respondent or is this the case for everyone with Android via WA?

Without knowing your server environment or the users setup, there is very little we can really say.

Thank you. It happened to everyone with WhatsApp via Android.
No idea why, but I just found a way to bypass the problem, by sending a shortened URL instead of the one authomatically generated by LimeSurvey.

Can you show a screenshot ?

OK : got it … sorry

Sure! 
However it's in Italian, and not more informative than what I already wrote.

No, it's me, I added and moved it up and down because I first got my reply rejected!

Yes,
Sorry ;: it's open inside whatsapp browser ? Right ?

In my opinion : it's a whatsapp issue … maybe related to ssl :
Check your url on www.ssllabs.com/ssltest/ or www.sslchecker.com/sslchecker

Thank you, I got the same impression!