I came across this while searching for a solution to the same issue. I am using version SoapUI 5.4.0.
It seems that SoapUI is holding ports open when an exception occurs. The only way to fix the issue is to Exit SoapUI or kill its process.
In my case the exception was caused by SSL Preference "Enable Mock SSL" being checked with a missing KeyStore certificate. SoapUI gave a not very helpful "...\SoapUI-5.4.0\bin ...File not found (Access Denied)" error when starting a mock service. I unchecked the option and all it resolved the issue (someone else installed SoapUI with the missing certificate specified).
