Automated tests do not run. Test cases go to state "In progress" and then shortly after to "Active"

We use TFS to trigger tests.

Build: Build TestComplete interface solution that connects test cases to TestComplete tests.

Deploy: Copies files to slaves.

Test: Distrbuted test. Connected to Microsoft Test Manager (MTM) Test plan. 

When queuing a new build, the Build and deploy part seems to work just fine.

The test part does not work. I can see in MTM that the test cases get the state "In Progress", but only for a second or two. Then they return to state "Active".

No log is posted to the test cases. No log posted to the build. The build finishes as Partially succeeded.

When I go and check the build log, I can see that 0 of 14 have passed.

When I log onto the computer where the tests are to be run, I can open TestComplete and the project without any problems.

We have 9 other systems like this that all work fine, and I don't see a difference when it comes to the build definition.

Has anyone seen this kind of issue before? I'm officially stuck.