SoapUI 5.0.0 not booting in Ubuntu (hangs on splash screen)

If SoapUI does not boot and hangs on the splash screen, the first step is to execute it via command line, to determine the error. Just

cd SoapUI-X.X.X/bin
./soapui.sh

In my case the error was

libXtst.so.6: cannot open shared object file: No such file or directory

but I already had the library installed. This thread in Stackoverflow put me in the correct path: JRE is a 32 bit app and is looking for the 32 bit version of the library, so I installed it

sudo apt-get install libxtst6:i386

Problem solved