This is a known issue for some time now.
This exception suggests that there is another element (div below the button) will receive the click.
1. Use another locator
You can try using the ThreadLocal Class, so each running thread would be assigned it's own webdriver instance.
See pseudocode to get the idea:
private static readonly ThreadLocal<IWebDriver> WebDrivers = new ThreadLocal<IWebDriver>();
public static IWebDriver Instance
if (WebDrivers.Value == null)