First let's check the notes section of the slide. The notes are actually part of the functionality and the URL in there must be correct. Frequently people will delete this, copy the poll image but not the notes, or do a search/replace and change the URL.
1. Click on the Poll Everywhere button at the top right of PowerPoint
2. Click the Log In button at the top left of PowerPoint and enter the login credentials of the user who created the poll questions
3. Click the Insert Poll Slide button at the top left of PowerPoint and select the poll you want to display from the window
4. Click Slide Show and From Current Slide. Does that particular poll slide display now?
5. Re-Insert any other Poll Slides that give this error.
6. *If you cannot find the poll in the PowerPoint window, you may have deleted it. Check your My Polls Dashboard to look for the poll.
If the notes field is not the issue, then the installation may have been corrupted. So, we will repair the file:
Completely close out of PowerPoint
Go to Control Panel > Programs and Features > Uninstall a Program
Right click on the Poll Ev Presenter
In the drop down menu, select the Repair option
Once the Repair finishes, open PowerPoint again and try to insert a Poll Slide
If neither of these work to solve the issue, contact Customer Support.