Same problem. Even weirder: It works once on the highest subdivision level, but once I drop down the resolution it stops working. Needless to say its BEYOND annoing.
I am running XP SP2 On an HP 2.40 Ghz dual core machine with 2 gigs of ram.
Its Zbrush 3.1 and its starting to make my face the color of the red wax mat cap.
Have either of you modified your interface? If so, what changes have you made?
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Just added the “double document” button in the UI so I dont have to go through the menue all the time. Its a clean install with no custom scripts or any of that high level mumbo-jumbo.
As far as my mem settings :
Compact mem 512
Doc undo 30
Tool undo 30
MaxPolymesh 6
HD Preview Max Poly 4
So, is it me or is it the unfavorabe star configuration for today?
I am using ZB 3.1. I’m running Windows Vista. I am still having this problem. The only modification that I have made to the UI is that I pressed tab a few time to hide and show some of the pallete.
Well, it seems there is some pattern to the undo problem. If I import and obj file as a brush, the ctrl z doesn’t work. But if I load a default sphere first, do some ctrl-Z on it, and than import the obj after that, it seems to work.
So Ill include this voodoo workaround in the pipeline doc for my artists in the hopes that at some point there will be a patch of some sort.
I’m also having the same problem in that the control-z will stop working. Also, after a bit the right control key and shift key on the right hand side will stop working and I have to re-start zbrush. I’ve noticed this when masking more than anything else. And, after this problem starts to show up using the control key on the right hand side of the key board actually starts to change which sub-tool I’m using on the tools palette.
I’m running the default V3.1 interface with no changes other than I’ve changed the memory preferences to 3GB and I’m running 8Gb.
If you have saved any custom hotkeys, in the ZBrush3\ZStartup\Hotkeys folder there will be a file named StartupHotkeys.txt. (If you haven’t saved any hotkeys, simply create this file in a text editor.) Try adding these lines to the file and restarting ZBrush:
[EDIT:UNDO,634]
[EDIT:TOOL:UNDO,634]
I’d be interested to know if this makes any difference to the problem.
Ah! Thanx a bunch Marcus! Seems to have solved the problem. 
Marcus,
Hadn’t saved any custom hot keys as of yet, still using the defaults for the majority of things, let’s just say I’m paranoid about breaking something. But, the suggestions for the text file seems also to have solved my problems. Many thanks.
Tom
Thanks guys, that’s great. 
thanks a lot marcus.
It worked.
Great.
It did not work for me. I have the MAC OS version ZbrushOSX 312B, and as of yesterday, zbrush was working like a charm. This morning my shift key, command key (trying to undo), and my tablet are not being recognized. I edited the Hotkeys files as recommended in this post, restarted the software and yet nothing.
Hey Marcus i just found this thread and the text fixed my problem for the undo 
But… do you have some text to fix the redo problem plz?
bilster666,
[EDIT:REDO,890]
[EDIT:TOOL:REDO,890]
should do it.
hi,
Would like to know if deleting the “hotkey.txt” file will force zbrush to recreate it? therefore fixing the problem?
similar to forcing maya to recreate its .env file
thanks in advance
would you happen to know the tags for resolution increase? my CTRL + D isn’t working.
No, it won’t. ZBrush doesn’t need the StartupHotkeys.txt file but it will use it if it is there. For most interface items Ctrl+clicking on the item and then pressing the hotkey you want to use will assign the hotkey (press Preferences>Hotkeys>Store to make sure your hotkeys are available every session). But the Undo buttons work slightly differently. It appears that there’s an internal issue which causes them to fail with the default set up. Assigning the hotkeys in the usual way doesn’t work because the buttons are given a number (e.g. ‘Undo 3’) and that gets included in the hotkey file, hence the need for my fix.
ToxinArrow,
I’ve not heard of that issue before. However, Ctrl+clicking on the Tool>Geometry>Divide button and then pressing Ctrl+D should assign the hotkey.
im running zb 4 on my MBP os 10.6.5 and my command key doesnt work ,i added the code and tried those tricks but still not solving the problem.
some one please help me with this,its really a pain in the neck
You have to turn your .obj file to Polymesh3D first. Then the Undo will work.
Step1. Open Zbrush , Take a sphere, make it polymesh 3d and test whether the undo
(CTRL+Z) and symmetry is working.
Step2. Now go to Zplugin click SubtoolMaster - click - MultiAppend and then Select your
Zbrush file (.ztl) and delete the sphere which u don’t want. Now you can use CTRL+Z
and symmetry
Hope this helps
Cheers