Perhaps the provisioning profile embedded in the build has been invalidated. Use Xcode to create a new archive, then use Fabric to upload a new build with that archive.
Discussion:
In my case, I had deleted the provisioning profile in the Apple Developer Member Center that had been embedded in each of my Fabric Beta builds. This caused the app testers had previously installed to immediately crash when they tried to launch it (embarrassing). It also caused the “Unable to download app – MyApp could not be installed at this time – Done / Retry” issue when testers tried to (re)install the app via Fabric Beta. Uploading a new build with my new provisioning profile embedded fixed the issue (each tester had to install the new build).