Please see https://github.com/mcneel/mono_embed for a sample to reproduce.
This sample embeds a 64 bit mono runtime and executes a method written in C#. The C# method in turn calls two exported C functions (one that takes an int and a double as parameters and one that takes a string).
On Yosemite / Xcode 6.4 this sample runs with no errors
On El Capitan / Xcode 7 this sample crashes when attempting to debug inside of Xcode. The crash occurs when trying to call the C function with a string parameter. When the Xcode debugger is NOT attached, this sample also runs with no problems on El Capitan.
Assigning to email@example.com since they have two other possibly related bugs. Please feel free to ignore my assigning guess however.