Instructions
Check that you are running the latest version of the Diablo 2 Hero Editor by clicking "About" on the main menu screen. As of January 2011, the latest version of the Hero Editor is V0.96. If you are not running this version, it can be downloaded from the ModDB website. Unzip the downloaded file and double-click "Setup" to install.
Update your copy of "Diablo 2" to the latest version, as the Hero Editor is not designed to run with outdated versions of the "Diablo 2" program code. Download the latest patches for "Diablo 2" from Blizzard's Diablo 2 website and then double-click to update the program. If the Hero Editor displays an error message after updating "Diablo 2," install the update as explained in Step 1. Even if you are running the latest version of the Hero Editor, re-installing will fix conflicts created by an updated "Diablo 2" game folder.
Start a new game by selecting "New" from the "Diablo 2" menu screen. Save your game by pressing "Esc" and choosing "Save," then quit the game. Open the Diablo 2 Hero Editor and click "Open" at the bottom of the menu. Navigate to your "Diablo 2" save-game folder (by default, this is "C:\games\Diablo II\save\") and double-click the newest file in the folder. This fresh save-game can be edited in the Hero Editor to provide all the enhancements and upgrades you want for your character, whereas save-files that have been saved multiple times or previously opened in the Hero Editor can become corrupted and unusable within "Diablo 2" or the Hero Editor.
Go through the list of enhancements you have given your hero, starting in the "Stats" tab and working your way through. After removing all the edits in a single tab, re-load your game in "Diablo 2." Once you find the tab whose edits make your game un-loadable, go through that tab, removing each edit until you find the one that has corrupted your save-file. Saving the file without this edit will make it loadable within "Diablo 2." As the Editor is prone to bugs, some edits can make your game inaccessible, but removing these edits will fix the problem.