I've had the same issue today, so I'm going to just put my outcome here for prosterity.
My error was slightly different from Manish's error. Mine ended with:
A schema validation error has been detected at line 0, position 0. Details: The element 'events' has incomplete content. List of possible elements expected: 'event'.
I did have to get some support from my Microsoft Partner on this one, so can't take the credit. However, it was an issue where I had accidentally made a change to my test environment that was meant to be made in my Dev environment against the offending form (which was identified through the GUID that is supplied in the error).
Here are the steps I took to rectify it:
1. In the environment to which you are promoting (in my case, I was promoting from Dev to Test - so this was carried out in the Test environment), open the Customize the System in the Customization area in Settings
2. Find the offending Form in its parent entity (which will be highlighted in the error message).
3. Select the Form by ticking next to it.
4. Select the "More Actions" drop-down and select Solution Layers
5. In this screen, you should see where the layer has come into the base D365 configuration by being edited in the Managed entity (this is because I had allowed customizing of a managed entity)
6. There should be a button to the right of the Solution name with 3 dots (only there if it has been completed in this way, not if you last upgraded through importing a managed solution)
7. In the 3 dots is the option "Remove Active Customizations", select this.
8. There isn't much in the way of messages or timing. I had to close down the window and re-open to see that the solution layer had gone.
9. I went back and checked, it had rolled back to the previous form.
10. I could then apply the upgrade without an issue.
I hope this helps someone else out who may have made this same silly mistake, and is now sitting at 3:22 am trying in vein to get an upgrade through with Google as their only lifeline.
