"Delete" key and Undo stop working

Hi, all-

I’ve been working from time to time at a studio that uses Nuendo, and I like it a lot, overall. I should also preface this by saying that we are currently using Nuendo 4 (yes, 4!) on a Mac Pro running OSX 10.6.8, and will be upgrading to Nuendo 6 in a few weeks, after first upgrading OSX to meet N6’s requirement. But in the meantime, I have a reeeeally pesky issue.

My problem is that from time to time, two very important things stop working: the “Delete” key does nothing, and “undo” cannot be performed. They always happen together, and they always come back together. There is some condition, either in Nuendo or elsewhere on the Mac, that is causing both of these to happen together.

At one point, I thought I could clear the issue by closing and re-opening the project, but that proved to be a red herring. It is also not a caps-lock issue or anything else with the keyboard, as both things malfunction regardless of whether I try to do them via keyboard or menu selection. Also, the Delete key continues to function in other programs while the issue is occurring in Nuendo.

Has anyone else come across this, or have any ideas to try? The inability to undo is really the bigger issue, as I can always “cut” instead (though not ideal). Thank you in advance for any light you can shed!