Access violation using a C++ function from a DLL

I have a game engine written in C++ which I'm porting to Windows (from Mac). It uses C++11 and OpenGL, and for all intents and purposes, it runs!

I'm using a DLL for my game engine which is linked implicitly to the game .exe at runtime. The problem is, when I try to use a utility class from the DLL, FileSystem, to find a resource file (a texture, but I don't think it's important), I get this error:

First-chance exception at 0x00007FF9CF988830 (PocoFoundation64.dll) in TestEquinox.exe: 0xC0000005: Access violation reading location 0x000000136A4FF000.

The problem comes when I call this method of my FileSystem class from the DLL (it's designed to take a filename/partial path and it looks in various places to find the full path):

Poco::Path FileSystem::Get(const std::string &filename) {
std::vector<Poco::Path> paths = {
        filename,
        ResourceFolder() / filename //<<<<< ERROR HERE
    };

    for (const Poco::Path &path : paths) {
        try {
            if (Poco::File(path).exists()) {
                return path;
            }
        } catch (...) { }
    }

    Logger("FileSystem", std::cerr) << "Could not find file '" << filename << "'!";
    return {};
}

Visual Studio shows the error as being at the call of ResourceFolder(), another method from the same class, also in the DLL. This appears so:

Poco::Path FileSystem::ResourceFolder() {
    Poco::Path userData;

    //--SNIP-- (other OS's #ifdef'd here)
    // GAME->StartupPath is a std::string containing the exe's parent folder
    userData = (Poco::Path(GAME->StartupPath).parent() / "Resources").makeDirectory();
    //--SNIP-- (and here)

    try {
        if (!Poco::File(userData).exists()) {
            Poco::File(userData).createDirectories();
        }
    } catch (...) {}

    return userData;
}

From the looks of it, it's to do with Poco's data types not being instantiated properly? I've built it from source with all the same compiler settings (64-bit, multi-byte character set, VS2013), so I don't see how it could be a name mangling/data layout issue.

One more thing to note - I copied the entire FileSystem class from the DLL to a class local to my game project, called FileSystem2. Calling FileSystem2::Get with the same parameters worked correctly and without crashing, despite being the same code.

Hoping someone can point me in the right direction?!