I was trying to run some XPerf traces to prepare for a training, when it all of a sudden stopped working. The error I got was this:

xperf: error: NT Kernel Logger: Cannot create a file when that file already exists. (0xb7).

Weird because I ran the same command successfully multiple times before. Trying to stop a potentially conflicting session by using:

Xperf –d blah.etl

Failed with this error:

xperf: error: Merge ETL: The specified path is invalid. (0xa1).

And the event log contained this:

Session "NT Kernel Logger" failed to start with the following error: 0xC0000035

What has changed was that I had started ProcMon.exe. That uses the NT Kernel Logger. Exiting that process cleared the way for my first command.