I couldn’t reproduce the issue here. Can you email me a sample file that I can reproduce the issue? (tech @ emurasoft.com)
Can you check the Maximum Memory Size in the Advanced tab of the Customize dialog box? The default value is 80%. You can also check the “Percentage of Physical Memory in Use” in the Status tab of the Customize dialog box. This “Undo will not be available…” message appears when the percentage of physical memory in use exceeds the specified percentage (80% by default).
If the message appears because the memory is actually low, you can set the “Prompt when No Undo Information is Available” or “Always Discard Lengthy Undo Information to Accelerate” check box in the Edit tab of the Customize dialog box.
Thanks,