Error -504 on appliation installation Yes. I've read all other questions regarding this problem.
Like this one:
Error 504 when downloading apps from Play Store?
It's the same problem as this one but with more background information and an other app:
How to fix installation error -504?

The details:
My device is an Ulefone Be Touch 2 running Android 5.0.

My connection is perfect. To be sure I tried different networks, among them are 3G, 4G and wifi on ADSL, VDSL, cable and fiber. I've also used Tor and 3 different VPN providers in 3 countries on 2 continents. Let's say I ruled the network out.


A single app does not install: OpenKeychain. 
Currently I have 126 apps installed.
It does not install via desktop browser > play store > install
button.   
I have enough free space (40GB). The app is 7MB.  
Re-adding Google account and clearing caches did not work.   
Even a complete device wipe and re-install didn't work.    
Downloaded the apk and tried manual installation, same error.


I also own a Nexus 7 (2013 wifi) and the app installs fine on that device.
I think this is a bug in Lollipop, I found this on another app forum:


  I’m getting the Error Code -504 (INSTALL_FAILED_UID_CHANGED) when installing App from Google Play
  
  Try to erase the “/data/data/appnamespace” directory on your device and try again. This is a “bug” Android 5.0 Lollipop developer preview which was fixed in the final release version.


That (plus a reboot, remove + reinstall of the app and remove + replace of my google account) fixed this issue for me.

It definitely isn't a connectivity issue, and it's nothing to do with a 504 HTTP error.