1. Download Wes' script from here:
Find and Delete Negative Keyframes
2. With Moho 12.5 open, select File > Open Custom Content Folder. if this command isn't available, it means you haven't set a Custom Content Folder yet. To set it, select Edit > Preferences. You can set the path at the bottom of the window. The Custom Content Folder can be anywhere you like; I suggest setting it somewhere local, like maybe in a 'Moho' folder inside your Documents folder so Moho always has access to it.
3. When you have the Custom Content Folder open, open the Scripts folder, then the Menu folder. Here, make a folder for custom scripts. I like to name mine '+Special', which forces the menu to the top of the menu list, but you can call it anything you like.
4. Put the syn_negative_keys.lua script in this folder. Now you can close these folders.
5. Quit Moho and restart it. This will add the new script. (Alternatively, you can press Ctrl+Shift+Alt+L...this reloads scripts without quitting Moho.)
6. Open a copy of the Project that's giving your trouble.
To be safe, make sure you're working with a copy!
7. Now you should see SYN: Negative Keys listed under the Scripts menu, inside the sub menu you created ('+Special' on my computer). Select the script, and it will do its magic. If it finds any negative keys, it will ask if you want to delete them. Say yes. (Understand that there is a possibility that this can mess up your Project file, which is why you should work with a copy. Chances are, everything will be fine though.)
8. Save your cleaned Project file and you should be ready to go.
Hope this helps.
Note 1: Under the Script menu, there is a command that tells Moho to install the script for you, but it requires that the script have a specific folder structure already set up. TBH, I'm so used to installing scripts manually (as described above,) that I almost never think to use this feature, but it's good to know about when the script author has a very specific setup in mind.
Note 2: the '+Special' folder is just something I do to keep things tidy. I actually go a step further by adding sub-folders by author inside that folder. You can use any structure that works best for you.
Note 3: the steps are the same for Moho 13.0 but the reload scripts shortcut is different. Frankly, I'd stick with Moho 12.5, and wait until Moho 13.5 comes out before setting this up for this version.