Deleting previous plugins didn't help.
What helped though: someone else was having similar problem but with v. 5.2.1: http://frmchandan.blogspot.com.au/2017/03/dependency-error-soapui-521.html
...so I added this dependency into pom.xml file:
<dependency>
<groupId>com.jgoodies</groupId>
<artifactId>forms</artifactId>
<version>1.0.7</version>
</dependency>and that fixed it :smileywink:
It might be helpful to note that this dependency should be added to the plugin dependencies and not the project dependencies in the pom.xml.
