Make sure you're using Chrome and that it's fully updated. You can verify/update if needed by selecting the 3-dots in the top right of your Chrome browser window. Then hover over Help and select About Google Chrome. The first field under Google Chrome will show you your version number or let you know if an update is available.

You also can try using Chrome's Incognito mode to see if that helps. If it does, then you may need to clear your browser cache or check to see if any browser extensions you have installed are interfering. 

(If you need help with clearing your cache please navigate to this How To link: https://help.playerlync.com/technical-faqs/how-do-i-clear-my-browser-cache)

If you're uploading files in batches, try just uploading one at a time. Here's a link to our officially supported file types:

https://help.playerlync.com/technical-faqs/what-file-extensions-are-supported-in-playrelync

Lastly try using a different WiFi network if possible to see if that helps. If your network is locked down, try reaching out to your IT team to see if they can help you.