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?
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.