Hey Jack,
You're right: this is a regression in version 1.5.6. Unfortunately, it seems I broke something. This is the software development version of the classical "two steps forward and one backward"

The CloneStamp tool works, actually. It's just that you must be careful when using it:
- Don't perform cloning actions very quickly. Act slowly to minimize the probability of failure.
- If the "locked for write" message appears, do the following:
* Press Esc to get rid of the message.
* Don't move the mouse at all, that is, don't use the mouse to click OK.
* Make one click over the image, to terminate the current clone operation. Otherwise it gets engaged, and you clone as you move the mouse. If necessary, undo the last action, if it is incorrect, pressing Cmd+Z (Ctrl+Z on Linux and Windows), or using the local history buttons on the CloneStamp interface.
Sorry for the inconveniences. I'll fix this problem in the next version. Mac users are lucky, since I'll travel with my Mac Pro during August, so I'll publish an update for the Mac version very soon

Linux and Windows users will have to wait until September, sorry...