QGIS » QGIS Application

I'll describe this step by step.

1. I want to simplify the geometries in a vector layer (ESRI Shape).
2. I click Vector:Geometry Tools:Simplify Geometries...
3. I select the vector layer in the drop down.
4. Without setting anything else I click OK.
   - QGIS responds with a message box:
   "Currently QGIS doesn't allow simultaneous access from different threads to the same datasource. Make sure your layer's attribute tables are close. Continue?"
5. I click the No button.
6. I check the "Save to new file" and select my new output file.
7. The state of the OK button is still disabled even though I have provided a valid output.

The same happens when I select an output in step 4, but the file I selected was locked by another program in which case QGIS complains. I then select a valid output instead but the OK button stays disabled.