In a powerpoint slide: If a placeholder value is set, but the placeholder doesn't exist in your slide layout template, the exported powerpoint slide will have unreadable content (and powerpoint will give an error). This scenario may be the result of setting a placeholder value, then changing the slide layout to a layout that doesn’t have that particular placeholder. Or it may be the result of removing a placeholder from the template, then re-importing the template.
Here’s a temporary way to fix it. For each slide, change the layout back to the old layout that has the necessary placeholder. Edit the placeholder value to delete the content. Then you can change the layout back to the desired layout.
To prevent the problem in the future, be sure to delete placeholder values before switching to another slide layout – if you think the new layout won’t have the same placeholders. (At least until the issue is resolved in RB).