I would try the following:

• From one of the other pilot environments, go to Customizer tab after logging in as a Customizer and export the specific UI Module you want, in this case Installed Item. Note where you save the exported file as you will use it for import.
• On the corrupted environment, log in as a Customizer and choose the Customizer Manager option on the far right.  You will have a view where you can expand the branches. Find the Installed Item branch and highlight it.  Once highlighted, use the X button on the top, try deleting the current customization for the module. I advise logging out again and then back in if it deletes. 
• If it deletes, you can then import the exported file from the first step by logging in as a Customizer, go to the Customizer menu and import the file.

I hope this works as otherwise you may have to go to the FileStoragePath for the profile, go into the customizations folder and locate the DEV branch, delete that folder to remove all pending developments.

Thanks for the reply Phil.  Can you clarify whether I should try this in the Public or DEV section?  I have previously tried this in the DEV section but I get an error message that the module is locked by the other user.  When the other user tries, they get a similar error message that I have it locked.

Should I try deleting in the Public section ?

It really does not matter which branch, Public or Dev as when it is deleted, it will delete all branches.  However, this was not successful for you so then I would advise the following:

• Export all customizations from your profile.  These will be only your Public customizations including what you have published for installed item as Public and not include QA or Dev branches.
• Goto your AsteaShare\FilestoragePath for the profile involved and copy the highlighted folders below somewhere for backup.  This would serve as a backup in case the below caused a problem for you.  (Shouldn’t as I did this myself as a test case)

• Delete the contents of the Customizations folder (you probably can’t delete this folder itself or at least I could not)
• Delete the CustomizationsHistory and CustomizationsLog folders
• IISRESET - the Customizations folder will have the config folder restored automatically but no contents.
• Log into the Alliance Browser as a customizer and open the Customizer menu.
• Import the customizations you exported in the first step.
• Recommend also to run the Customizer - Conflict Resolver just in case.

Now you should have all your Public customizations back but no Dev branch which is what you wanted as your screen only shows the one module in the DEV branch anyway for the UI Screen.

After the import, your application will be forced to restart and this should resolve your issue. You can create a new customization from that point but of course don’t let the two users edit the same module without publishing it to Public again…

Much appreciated Phil, this did the trick!  We will definitely be more careful going forward.