This happens when the part you want to edit in place is already opened in solid edge; close it and the edit in place function will back to work.
I'm having exactly the same issue, but do not have the part open (prior to editing in place - you can see this in the images as the Switch Windows button is grey'd out). I'm finding so many little issues like this since upgrading to ST8...! I have the latest Maintenance Pack installed (#5).
See images below:
This seems to be just the "Hide Previous Level" has been selected (maybe mistakenly using the shortcut Ctrl Q).
Thanks very much for your prompt attention, really appreciate it. This has solved our issues. None of us were aware of this shortcut.