Just because you are running Ubuntu does not in fact mean that you are not running SELinux... or something similar. Packages like SELinux or AppArmor are packages that implements access control in linux. These types of packages have been known to cause issues with osTicket.
regarding 1.
how many departments do you have?
running out of memory seems to be the issue. I've never seen someone run out of memory on going into departments before. What is department 3? Since the url indicates 3 as causing the error. You may have to access the database directly to see what the department is.
regarding 2.
RBGE told you how to do that 6 days ago.
Regarding 3
cgi.fix_pathinfo 1 is for windows. You are not running windows/IIS and can ignore it.