The problem, as explained in http://www.slf4j.org/codes.html is that one (and only one) of the following is missing in the classpath: slf4j-nop.jar, slf4j-simple.jar, slf4j-log4j12.jar, slf4j-jdk14.jar or logback-classic.jar.
I found that adding, e.g. slf4j-simple.jar to [SoapUI-Base]/bin/ext did not work, even though it apparently loaded it:
13:02:26,203 INFO [SoapUI] Adding [C:\Program Files\SmartBear\SoapUI-5.2.1\bin\ext\slf4j-simple-1.6.1.jar] to extensions classpath
13:02:26,983 INFO [DefaultSoapUICore] initialized soapui-settings from [C:\Users\XXX\soapui-settings.xml]
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
What worked was to place slf4j-simple.jar, and more specifically, slf4j-simple-1.6.1.jar (to be compatible with the SLF4J version used in SoapUI 5.2.1) into [SoapUI-Base]/lib.
Hope this helps.