Second XLS Decision Table upload fails

Description of problem:
First Spreadsheet/XLS DT is created and uploaded just fine. Trying to create and upload another one right away results in error pop-up with HTTP Status 500 (see screenshot) and exception stacktrace in server log (see the other attachment).

Version-Release number of selected component (if applicable):
6.0 ER1

How reproducible:

Steps to Reproduce:
1. Open the business-central, Project Authoring, some group, repository, project and package.
2. Click New Item and choose Decision Table (Spreadsheet), type some name and select some XLS file to upload. Click OK.
3. Dismiss the alert saying that the upload was successful.
4. Repeat step 2 with a different name (and optionally different file).

Actual results:
"Upload failed: <h1>JBWEB000065: HTTP Status 500 - No enum constant org.guvnor.common.services.shared.file.upload.FileOperation.</h1>...."

Expected results:
Succesful upload of the second file.

Additional info:
1. The problem can be avoided if you relog between the uploads.
2. I find it curious that a problem with file upload experienced while working with XLS DTs has "at org.jbpm.designer. ..." in the exception stacktrace.
3. The error pop-up will be filed as a separate BZ.