Thanks for your reply, as it means I can post one of my own - I was
logged out immediately after posting my AdobeID wasn't being recognised
so I couldn't edit my own post to add more detail...so I can now, but
anyway don't get my started on that.
So yes I know more information is needed. As for searching for
vagueries, there's a big difference between "Blue Sky" (very good anime
movie) and "Forum poster ate my dog and spat it back in my face,
site:adobe.com".
Here are the specific events in order, over a period of the past week:
1. Selecting a layer.
2. Pasting a 512x512 image from clipboard into a layer
3. Dragging an image from disk to the canvas
4. Selecting a layer
5. Trying to apply guassian blur filter (error on selecting the panel.
6. Selecting a path.
This is in Windows 8.1, working on RGB 8bit/channel documents with a
reasonable number of layers (>50,nested). There's nothing special about
the documents, no TypeKit fonts or Smart/Linked Objects (exception being
#3, trying to drag an image from desktop to the active doc).
All documents affected were less than 4,000 x4000 pixels, and the errors
only affected the active document - other open documents could be saved,
even edited, and closed safely.