Ok, so one good news and one bad news.
Good news: After using 1.5.1 for the build, and ensuring the cache was emptied, it now works on the iPhone6. And the gallery is nowresponsive on the devices I have tested.
Bad news is that the pop-up "JuiceboxBuilder-Pro wants your confidential information stored in "Adobe.APS in your keychain"-issue still remains after following the tips you referred to.
The files referred to does not even exist on my computer:
~/Library/Application SupportAdobe/AIR/ELS/JuiceboxBuilder-Pro
~/Library/Preferences/JuiceboxBuilder-Pro
And none of the other solutions helped either.
I am now running Adobe AIR 25 instead of 30 and still get the pop-ups.
Current status: If I click Deny in the pop-up appr 10 times through the workflow of Juicebox Pro the application actually delivers the gallery files at the end. As this is not a desirable workflow, I look forward to further ideas on how to remove this problem.
Again, thanks for the very prompt support!