Xcode service - CI Bot error when performing Unit Tests

When an Xcode Service bot attempts to perform a build with 'run unit tests' enabled, the build always fails. When looking at the log file the same error repeated many times:

xcsbuildd[1966] <Warning>: [XCSBuildOperation.m:106 7368f310 +2126ms] Preflight connection to /var/XCSControl/com.apple.XCSTestUserPreflightService failed; retrying: No such file or directory

Followed by these messages a few lines later:

xcsbuildd[1966] <Error>: [XCSBuildOperation.m:102 7368f310 +2104ms] Timed out trying to connect to CG Session; giving up after 60 seconds

xcsbuildd[1966] <Error>: [XCSOperation.m:33 7368f310 +0ms] Error Domain=XCSBuildServiceDomain Code=0 "Cancelled integration because headless test environment is required but not available after 60s" UserInfo=0x7fa952571bd0 {NSLocalizedDescription=Cancelled integration because headless test environment is required but not available after 60s}

xcsbuildd[1966] <Debug>: [XCSOperation.m:28 7368f310 +0ms] Cancelling operation: XCSBuildOperation

I've hit a brick wall with this, and can't find anything on it.

Has anyone had this problem and found a fix?