Alexey Bychko I've uploaded an enhanced hudson-wsclean-plugin.hpi file. Try ticking the "use history" check-box (in the "Advanced" config). That'll change the plugin's behavior from "hit all slaves that could run this project except the one running this build right now" to "hit all slaves that previously ran this build except any running this build right now". That should allow you to have concurrent builds without a problem. Note: By default, it'll continue to use the old "hit all slaves that could've run this project" logic which is largely unchanged and will still bork a concurrent build - you'll need to select the new behavior in your job config.
If you still have issues, create a log (manage jenkins -> system log) that logs de.jamba.hudson.plugin.wsclean at level "ALL", collect logs from that to see what it's doing wrong (as this code now logs what it's doing to the Jenkins logger as well as to the builds), and let me know further details.