You can use
Advanced - Backup/Restore User Customizations and backup at least
uedit32.ini, better would be to backup all.
Then exit UltraEdit and delete
%APPDATA%\IDMComp\UltraEdit\uedit32.ini or just rename it to something else. Or you rename the entire directory
UltraEdit to something different. Start UltraEdit which creates
uedit32.ini or the entire directory
UltraEdit completely new.
Test now undo feature. Is it working again?
Yes, then exit UltraEdit, create a copy of new
uedit32.ini, start UltraEdit again and open the copy of
uedit32.ini and the old
uedit32.ini with not working undo feature. Use script
RegIniFileSort - Script for smart sorting REG and INI files or macro
Sort UE/UES INI from
Macros for smart sorting INI and REG files to sort both INI files.
Now it is possible to compare the 2 INI files and merge everything different or missing from old
uedit32.ini to copy of new
uedit32.ini which is definitely not responsible for not working undo feature (scripts list, user tools list, theme, ...).
Then exit UltraEdit and replace
uedit32.ini with copy of new
uedit32.ini updated with the settings from old
uedit32.ini. Start UltraEdit and check if undo feature is still working.
If undo feature is not working even with completely new created
uedit32.ini, restore previous
uedit32.ini respectively delete new
uedit32.ini or directory
UltraEdit and rename back old
uedit32.ini respectively
%APPDATA%\IDMComp\UltraEdit. Then uninstall UltraEdit completely via Windows Control Panel (user configuration in application data directory is not changed) and install UltraEdit again.