Hello Mate!
The t3-assets forlder is generated when uses ja template based on the t3 version 2 , this is where storing cache of js, css files if the template enabled the optimization function.
Your reposting issue, its seem be related to the permission on the file, folder:
1) May be your server configured and did not allow access some folder have created.
2) Or the file and the t3 folder have wrong permission, you try to set permission 755 for folder and 644 or 755 for all css, js cache file
=> Then check this issue again, and Please provide me the administrator account , FTP account if the issue is not fixed
Here is your ticket ID CFA-521-56032
Thanks