Is there a successor to CSipSimple? The goal is to use the CSipSimple on a TMO Galaxy S5.  I found the .apk files, however they are dated 2012:

https://code.google.com/archive/p/csipsimple/downloads

CSipSimple-0.04-01.apk installed but returns an error at boot up:


  Can't load native library CPU arch invalid for this build



Is there a simple fix to overcome the error message?
Is there a successor (fork?) to CSipSimple?
Is there a version of CSipSimple that is Marshmallow compatible?


Ideally there would a site that stores more recent .apk files.
I don't believe so. CSipSimple was a project started by Régis Montoya in 2010, the last stable build was in 2014, and it was abandoned in 2017... It was removed from the Play Store for download mid-2018. Source

The issue you are experiencing here isn't (likely) the Android version, but the architecture. CSipSimple was never made to support a 64-bit OS or instruction set and it only supported a 32-bit arm environment. CSipSimple was not compatible with any device other than "arm" architecture, and was not compatible with arm64 or other platforms. 

To answer questions specifically:


  Is there a simple fix to overcome the error message?


Yes... Install the application in a device with compatible architecture. Otherwise, no.


  Is there a successor (fork?) to CSIPSIMPLE?


To my knowledge there is no SIP softphone that officially states it is successor of CSipSimple. My guess would be that several SIP softphone software on the Play Store is likely based on, or uses some code from, CSipSimple's code base as it was one of the first stable generic SIP softphones out there for Android. Application recommendations are outside of the scope of Android Enthusiasts. 


  Is there a version of CSIPSIMPLE that is Marshmallow compatible?


As I stated earlier, the OS version is not the issue in this case. I can tell you from personal experience it runs on Android Oreo just fine as long as it is a 32-bit OS, so Marshmallow compatibility shouldn't be an issue. 

Although it is beyond the scope of this forum, you could download the source code and attempt to modify it yourself. It is still available in the Google Code Archive here.