Maven, SoapUI-plugin and external Groovy scripts

I am currently trying to migrate our soap ui-tests to run with maven.
Unfortunately my external groovy scripts (in <project-base>/src/test/groovy) are not found during the test execution.
The folder contains multiple scripts within their different packages / directories.

My pom-file (excerpt):

<!-- SOAP-UI-Settings -->
<properties>
 <!-- References an empty settings file - Maybe this solves the issue that the scripts aren't found?! -->
 <soapui.settings.empty.file.path>${baseDir}/src/test/resources/empty-soapui-settings.xml</soapui.settings.empty.file.path>
</properties>

<build>
 <testOutputDirectory>target/test-classes</testOutputDirectory>
 <scriptSourceDirectory>src/test/groovy</scriptSourceDirectory>
 <testResources>
  <testResource>
   <directory>src/test/resources</directory>
  </testResource>
 </testResources>  
 
 <plugins>
  <!-- Run SOAP UI tests during the integration phase. -->
  <plugin>
   <groupId>eviware</groupId>
   <artifactId>maven-soapui-plugin</artifactId>
   <version>4.5.1</version>
   <configuration>
    <testFailIgnore>true</testFailIgnore>
    <settingsFile>${soapui.settings.empty.file.path}</settingsFile>
    <outputFolder>${project.build.directory}/soap-ui-reports</outputFolder>
    <junitReport>true</junitReport>
    <printReport>true</printReport>
    <exportAll>true</exportAll>
    <soapuiProperties>
     <property>
      <name>soapui.scripting.library</name>
      <value>${project.build.scriptSourceDirectory}</value>
     </property>
     <property>
      <name>soapui.logroot</name>
      <value>${project.build.directory}/soap-ui-reports</value>
     </property>
    </soapuiProperties>
   </configuration>
   <executions>
    <execution>
     <id>project</id>
     <configuration>
      <projectFile>src/test/soapui/project.xml</projectFile>
     </configuration>
     <phase>integration-test</phase>
     <goals>
      <goal>test</goal>
     </goals>
    </execution>
   </executions>
   <dependencies>
    <dependency>
     <artifactId>jdbc-oracle</artifactId>
     <groupId>jdbc</groupId>
     <version>10.2.0.3</version>
    </dependency>
   </dependencies>
  </plugin>
 </plugins>
</build>

Within the log-file the plugin writes the info that the directory is used:

Setting soapui.scripting.library value D:\workspaces\soapui-it\src\test\groovy

But I always get the following error:

Script1.groovy: 1: unable to resolve class com.company.ExpiresChecker
 @ line 1, column 1.
   import com.company.ExpiresChecker
   ^
org.codehaus.groovy.syntax.SyntaxException: unable to resolve class com.company.ExpiresChecker

Does anyone now what's wrong up there?

Thanks.