HTTP 500.19 and 500.21: IIS refuses to read your web.config at all
HTTP 500.19 and 500.21 mean IIS cannot parse or is locked out of web.config. Unlock the section, fix permissions and the site loads on the next request.
HTTP 500.19 on Exchange virtual directories: broken IIS configuration files
HTTP 500.19 means IIS cannot read the web.config for an Exchange virtual directory. Learn how to reset the vdir and revive its application pool.