>> Keep in mind that if I install Aqua Mail first, then I cannot install Pure Messenger... So it may not be his fault completely
Yes, I got that part.
Aqua declares a permission for third party apps, and this permission protects access to Aqua's account list (and messages too), for security reasons.
That's all Aqua does.
I just purchased and and tried installing Pure Messenger on my Nexus 5 with 5.0, and got same error.
I was able to install my own Sony Smart Watch app that also uses the same permissions.
We'll just have to wait for Francois to respond.