Error: Problem capturing screenshot: null

Today I got the following error from our Selenium RC server when trying to capture a screenshot using PhpUnit's captureScreenshotOnFailure setting:

RuntimeException: The response from the Selenium RC server is: ERROR:
  Problem capturing screenshot: null

Reason for this were the file rights on the Selenium Grid node; the directory for the screenshots did not exist. You will also get the same error message when the directory is not writable by the client.