The reason for this error is that you have to define explicitly in the pom.xml that the maven-release-plugin should use svnkit.
Here's how:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-release-plugin</artifactId>
<version>2.4.1</version>
<configuration>
<goals>clean install</goals>
<providerImplementations>
<svn>javasvn</svn>
</providerImplementations>
</configuration>
<dependencies>
<dependency>
<groupId>com.google.code.maven-scm-provider-svnjava</groupId>
<artifactId>maven-scm-provider-svnjava</artifactId>
<version>2.0.5</version>
<scope>compile</scope>
</dependency>
</dependencies>
</plugin>
</plugins>
</build>