Xcode bot exit without completing

I've been getting semi random instances of

Test operation failure: Test session exited(-1) without completing.

Errors on my Xcode CI bots. I haven't got a clue on how to fix this. These has been attempted without any positive result

• Cleaning the DerivedData folder of the Xcode build server – all folder named DerivedData within /Library/Developer/XcodeServer
• Restarting the server machine

Any ideas how to fix this?

Some environment:

• Xcode 6.3.2
• iOS SDK 8.3
• OS X 10.10.3
• Server 4.0.3