TFS 2015 Test Execution: Failed to initialize client proxy: could not connect to test process

I have just upgraded to TFS 2015 and am trying to get our builds working correctly. I've got everything running except the unit tests, which are failing with the same error message in both my legacy XAML build definition and a new web-based build definition I have created.

From what little I've been able to find about this issue online, it has to do with the user vstest.console.exe is running being denied access to named pipes, but I can't find any information about how to change this.

I've tried running vstest.console.exe manually in a command line window against my unit test DLLs, and it works fine.

XAML Build Defintion:

TF900546: An unexpected error occurred while running the RunTests activity: 'Failed to initialize client proxy: could not connect to test process .'.

Web-based Build Definition:

2015-12-06T06:12:55.2314714Z Warning: Using Isolation mode to run tests as required by effective Platform:X86 and .Net Framework:Framework35 settings for test run. Use the /inIsolation parameter to suppress this warning.
2015-12-06T06:13:55.6697450Z ##[error]Error: Failed to initialize client proxy: could not connect to test process .
2015-12-06T06:13:55.6697450Z ##[error]
2015-12-06T06:13:55.6697450Z ##[error]Error: There was no endpoint listening at net.pipe://myservername/TestExecutor/5960 that could accept the message. This is often caused by an incorrect address or SOAP action. See InnerException, if present, for more details.
2015-12-06T06:13:55.6697450Z ##[error]
2015-12-06T06:13:55.8416222Z ##[error]VSTest Test Run failed with exit code: 1