Just to make sure we are talking about the same place... (I also assume you are on Windows. Does the Mac even use these .dlls?)
I have the file in the following location on my harddrive:
C:\Program Files\OpenToonz 1.1
I see I also have a glu32.dll at C:\Windows\System32 (note the different spelling)
But I believe that is a different and unrelated thing.
One thing you can also do is make sure you fully Uninstall OpenToonz before installing a new/current release.
If for any reason that file is corrupted uninstalling should make sure there is nothing in that file location so a new one will be installed in its proper place.
For completeness sak... in the portable release the glut32.dll is in the same directory as the Opentoonz program also.
If I remove/rename the dll OpenToonz fails with a message similar to what you state although even before the program opens.
Restoring the file resolves the problem.
Also, you say that this happens when you try to open 'an animation'.
By that do you mean a .tnz file or some other thing (such as a video).