The official xposed-v78-sdk21-arm.zip fixes any issues with v21a. So go to the official thread, download, flash and it just works:
The user androphone on Android-Hilfe has figured out how to install Xposed on v21a:
http://www.android-hilfe.de/thema/xp...k-v21a.733310/ (use google translate)
Quick translate (TRY IT OWN AT YOUR OWN RISK!)
0. As always make an BACKUP (important!)
1. Deactivate „LGSmartCover“! (freeze with Titanium Backup!) otherwise you'll get a bootloop.
2. Get the inofficial „xposed-v75-sdk21-arm-arter97-snapdragon“ made for Samsung devices!
3. Flash it in your recovery. → Be carefull - the first reboot will take really long (at least 5 minutes)
4. After it successfully rebooted install „XposedInstaller_3.0_alpha4“ and after that your modules as usual
I'm using „G3 TweaksBox“, „True Silent Mode“ and „GravityBox (LP)“ without any issues (on v21a)
If you get an error message, that you can't close - note the name of the app, remove your battery, reboot into recovery, reflash your backup, reboot your system and freeze the noted app the same as „LGSmartCover“! and restart with step 1.
All credits go the original author (androphone)