This is a known issue and is in the bug tracker. When you use either "Render Still" or "Render Animation" from the "Render Controls" section of the crowdrender panel, the render engine can set set to a blank value.
The fault is shown In the image above. The "Render Controls" panel is showing a red stop sign and reporting that there is no render engine selected.
It is still possible to render in this case, all that has occured is that our background engine is still selected after a render. You can fix this problem by simply going to the render engine drop down menu in the info bar and selecting the engine you wish to use!
When it will get fixed
This bug will be part of the next development sprint, this means it should take no more than a couple of months, which is our developemnt cycle at the moment.