Now that you are not running the version from github...
Try logging in as an agent with Task permissions and clickig on the Tasks tab at the top of the screen. What happens?
Now click on New Task. What happens?
Press f12 and see if there are any console errors.
Also check your PHP/Apache error log.
Now repeat this by viewing a ticket.
Click on the Tasks tab embedded in the ticket.
Click on the "Add New Task" button on the right side of the area under Tasks.
What happens?
Press f12 and see if there are any console errors.
Also check your PHP/Apache error log.
Post any errors that you get in console, or logs here. Please note which way you were accessing tasks when with the error.