Problem: Viber Desktop does not start on Debian Linux.

When we running the command /opt/viber/Viber in the terminal, we can see the error:

/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.15' not found (required by /opt/viber/Viber)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/Viber)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Widgets.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.15' not found (required by /opt/viber/libQt5Network.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Network.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Sql.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5WebKit.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5WebKitWidgets.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Declarative.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5DBus.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5OpenGL.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Gui.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.15' not found (required by /opt/viber/libQt5Core.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Core.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libicui18n.so.48)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libicuuc.so.48)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Quick.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Qml.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5PrintSupport.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5XmlPatterns.so.5)
/opt/viber/Viber: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.14' not found (required by /opt/viber/libQt5Script.so.5)

what to do?

The idea was found on superuser.com:

go to packages.debian.org, download fakeroot and install from individual packages:

mkdir /opt/fakeroot
dpkg -x libc6_2.19-11_amd64.deb /opt/fakeroot
dpkg -x libc6-dev_2.19-11_amd64.deb /opt/fakeroot

Now, start with new command:
LD_LIBRARY_PATH=/opt/fakeroot/lib/x86_64-linux-gnu/ /opt/viber/Viber

Profit!