Error trying 'COPY POSTS' to a New Topic

I just selected (ticked) 17 posts spanning 2 pages from another thread and then tried to start a new topic with them and got this error. Not that I did "Copy to a New Topic" and not Move to a New Topic".

labels.PNG

When I click the X to close window then got this next window that just keeps showing the spinning icon but doing nothing else.

Then when I clicked the X again to close that window see there is a language variable missing.

Just to say that I did Copy to a New Topic by mistake really when getting this error. I meant to do Move to a New Topic instead, which I have just tested doing again and that worked fine. So seem related only with: Copy to a New Topic.