Without any more evidence I suggest the following:

Grep the sources for the error message. You might even want to change it slightly (add dash, dot or something), recompile and verify that you found the right spot in the source. Then scroll backwards in the source code and see what was tested. Most probably there is a quite clear "if(some system call(supplied path))" a few lines before the error message. Then you need to figure out why it is returning something that causes the error message. You may want to add a log line that prints the supllied homedir and the effective uid to the logs to verify that they indeed are what you expect.

This is the way I have debugged some mysterious error messages in open source code. Usually I have found out that my own stupidity outsmarts me by large. I am not a C-genius, there are ample examples of logging in the code. Just cut, paste and edit a bit. Be careful with the parenthesis and semicolons, they can cause even more mysterious output from the compiler.

You might even try to create your own "Hello world" type of program that calls the same system call with a fixed string and prints the output. The only challenge is to find which header files and libraries to include, but man pages are your friend.