After upgrading to Marshmallow (Resurrection Remix) on S4 GT-I9500, the xposed apk file was not installing.
I had followed all instructions, proper .zip and .apk files, even clearing cache, etc. but nothing was working.
However, I found this solution on the net and it works!
xposed is active now.
After trying to post the message, I realised that since I am new to the forum I am not allowed to post external links.
But google "xposed app not installed" without the quotes and you will see the solution on a website (may be 4th or 5th from top) called mrad4tech.blogspot
This is what it says (credit to original author):
(1) Root enabled ES file explorer required
(2) Copy the not working .apk to system/app (retain the .apk file in original location for step 6, if required)
(3) Reboot phone
(4) You will see the application in the app drawer
(5) Try opening the app (this worked for me so I did not have to go to next step)
(6) If it force closes then install the .apk (in original location) now normally and it will work