Redirect and other issues when logging in to Fortigate 100Ds

Hi!

I have several 100Ds that experience quite similar issues. When I log in to them locally (Switch->MGMT port) I have no problems at all. On the other hand, every time I want to sign in from outside (through port 4443 (I changed the standard one)) I observe the following behaviour:

- I get kicked out randomly - usually my sessions are no longer than 1-3 minutes or less (Timeout time is set to 15minutes)

- When I try to login and password is correct it still redirects me to the login screen at least 1-3 times:

https://170.X.X.X:4443/login?redir=%2Fng%2Fprompt%3FviewOnly%26redir%3D%252Fng%252Fprompt%253FviewOnly%2526redir%253D%25252Fng%25252Fpage%25252Fsystem%25252Fstatus%25252Fstatus

- I tried using Chrome, Firefox, IE - same result. Also, I tried logging in from different providers

Firmware: v5.4.0, build1011, 151221

I would appreciate any help on this matter.