I spent hours trying to solve this bug and the solution was so simple. 
My rpcpassword in my bitcoin.conf file contained a '#' character.

Prevent this from happening, by outputting this an error or warning when users run bitcoind.

Right now, you can have a # character as a password and it will work on the command line but will fail when you JSON-RPC.
{
    try
    {
        return fs::create_directories(p);
    } catch (const fs::filesystem_error&) {
        if (!fs::exists(p) || !fs::is_directory(p))
            throw;
    }
    
    return false;
}
bool FileCommit(FILE *file)
{
    if (fflush(file) != 0) { 
        LogPrintf("%s: fflush failed: %d\n", __func__, errno);
        return false;
    }
#ifdef WIN32
    HANDLE hFile = (HANDLE)_get_osfhandle(_fileno(file));
    if (FlushFileBuffers(hFile) == 0) {
        LogPrintf("%s: FlushFileBuffers failed: %d\n", __func__, GetLastError());
        return false;
    }
#else
    #if defined(__linux__) || defined(__NetBSD__)