The fix for me was to add an xml-security dependency to the soapui maven plugin like so:
<!-- To run, enter this command from the module directory: mvn soapui:test -->
<build>
<plugins>
<plugin>
<groupId>com.smartbear.soapui</groupId>
<artifactId>soapui-maven-plugin</artifactId>
<version>5.1.1</version>
<dependencies>
<dependency>
<groupId>org.apache.santuario</groupId>
<artifactId>xmlsec</artifactId>
<version>1.5.2</version>
</dependency>
</dependencies>
<configuration>
<projectFile>src/test/soap-ui/your-soapui-xml-file.xml</projectFile>
</configuration>
</plugin>
</plugins>
</build>
In my case, I didn't have an xml security dependency anywhere else in my project.
In other cases, it seems like some may be experiencing this error due to having multiple xml security dependencies throughout their project (and they are conflicting). So, if the above fix doesn't work for you in that scenario, you may need to look into which depedencies in your project are including xml security and deciding which one you want to include and which one you want to exclude (using maven's exclusion or provided markings in the given pom).
