Apparently some older spreadsheet URLs are redirecting to newer ones, and the redirect process is dropping some security related headers that are required for the current way queries are being done to be more secure and faster (using XHR). If you look in your browser's debugger console, you should be able to see the redirect in an error message, and replace the URL in your query with the new URL.
I am debating whether to rollback the v49 release (so 'current' would again be '48') or just let everyone who needs to switch to '48'. This won't be easy to fix from the Google Charts side, and the redirects on the spreadsheets side may also be difficult if not impossible to change. At best, we could check whether the URL looks like it requires the older query mechanism, which may fail on IE11 or Edge (which was one of the motivations for the change).