Close TurboTax, then try clearing your browser's cache and deleting your cookies. See the TurboTax Help articles How to clear your cache and How do I delete cookies? for guidance.
Then, close the browser, re-open it, and open TurboTax Online again.
Finally, make sure you're only downloading information from one brokerage at a time.
If these suggestions aren't helpful, see the following TurboTax Help articles for potential fixes:
