can't access shared files

Hi Mostafa, 

Welcome to Box Community!

Here are some troubleshooting steps to try:

1. Make sure that the browser is up to date
2. Clear the browsing history or use a different browser
3. Temporary disable plugins/extensions, firewall, and VPN
4. Check if your Firewall is configured for Box apps 
5. Try to connect to a different network
6. Reach out to the owner and ask to remove and re-invite you to collaborate
 
I trust that the above steps will do the trick. Let me know how it goes!