I have same problem and I had solved that.

1. Place your cursor in the place caption will be created
https://i.stack.imgur.com/NUznQ.png

2. CTRL + Shif + Alt + S to show Styles dialog.
https://i.stack.imgur.com/7bTW0.png

3. Click "Caption"and click modify
https://i.stack.imgur.com/AXZf5.png

4. Make sure the Style is "Normal" and tha alignment is "left" and then click "OK"
https://i.stack.imgur.com/YVHBB.png

5. Now you can insert caption and can edit text in caption box.
https://i.stack.imgur.com/AihLg.png

Hope this will help.