Undo Tool not working

Anyone know how to make the undo tool work again?

Are you using the keyboard shortcut or the undo at the bottom left of the model window? What browser are you using?

Sorry, I should have been more specific.
I was referring to the undo icon in the bottom left corner. The shortcut (Ctrl + Z) does work.
I am using Firefox

Still is working for me. If you choose Firefox/Tools/Web Developer/Web Console, you might look to see if any errors are being reported.

Seems there was an XML parsing error. Was not saving my project either. Had to shut down and restart the browser and it is now working.

Thanks for the report. Any idea what XML file had a problem?

No. Sorry, I should have copied the error. I will do that if I see it again.
Thank you for the info.