This is a know issue, frequently discussed in this forum.
The Muse team has already fixed this error. If you want to try, sign into www.museprerelease.com and download the prerelease version (the prerelease version won’t conflict with the Muse version you are using).
As a workaround you can do the following:
Start Muse and immediately click onto the Finder background or a Finder window to take Muse out of focus. Muse will start now correctly.
This Issue is NOT Fixed. I have the latest version of Muse and the problem persists. I have fully uninstalled and reinstalled Muse several times, I have used Adobe's clean tool, the Adobe help team (oxymoron) have attempted to do the same thing and have deleted and reinstalled without success, only to tell me to email the tech team for support and NOT provide an email address.
I finally got the Muse program to finish loading (without Adobes Help) and checked that it was version 2017.0.1.11 and, after several trys, it successfully opened and converted an older Muse file. I then saved and closed the program. When I attempted to Start Muse again, same problem occurred and Muse would not start. I tried about 6 or7 times and then it finally opened again, showing my updated files ok. But then after closing, when attempting to reopen, it again failed to completely start. This time, despite about 20 attempts, it is still not opening.
THIS PROBLEM IS NOT FIXED, it still persists and Adobe appear uncaring about fixing the issue.