Cannot execute SCM from Ant build script while RTC is running, message is "Another RCP application is running in this sandbox", exit code 27

Hello,

I'm running SCM command from within my Ant build script, something like this:

        <exec executable="${scm.home}\scm.exe" failonerror="true">
            <arg value="checkin"/>
        	<arg value="myProject/build/run.client.test.sh"/>
        	<arg value="-u"/><arg value="…user name…"/>
        	<arg value="-P"/><arg value="…password…"/>
        </exec>

This build script is launched from RTC client and fails with following message:

Another RCP application is running in this sandbox. File locked at file:D:\Workspaces\rtc.workspace

And <exec> returns code 27.

I ultimately need to run this script not in a stand-alone manner, but from RTC. If I deregister the sandbox reported as locked from RTC Eclipse daemon, it will no longer show pending changes which is definitely not what I want.

Any suggestions how to work around this?

Thank you!