Why can't I post a YouTube link in What's App?

I am using an old Iphone 4,
and have just started experiencing the following problem...
When I copy a YouTube link into the message section of
What's App, my phone instantly exits What's App and
goes to the Iphone homepage.
When I re-enter What's App, the message & link have
been deleted.
I have tried re-booting the 'phone, but the problem remains.
Is it possible my 'phone has a virus>.
Would appreciate any advice, please...

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