You may need to clear your browser's cache and delete the cookies stored on your computer. A full or corrupted cache can keep TurboTax from functioning properly.

Please see the below FAQ for instructions on how to clear the cache and delete cookies.  

Clear Cache:  https://ttlc.intuit.com/community/troubleshooting/help/how-to-clear-your-cache/00/26135  

Delete cookies: https://ttlc.intuit.com/community/troubleshooting/help/how-do-i-delete-cookies/00/25978

Also, try refreshing your browser and even try using a different browser if the issue persists. If none of these resolve the issue, please contact Customer Support for further assistance.

I have tried the suggested: clear browsing history and clear cookies ... neither worked.

I am using Windows 10 and the Edge browser.

Also I find that a separate, initial TT 2020 file DOES OPEN, but my most complete and recent file consistently shows the 25300 error code and is unresponsive.

For possible causes and troubleshooting steps for error code 25300 please click the link below.

Error 25300