Refresh page redirects to home page

Hello all, i have a weird problem in my test environment ( currently using zextras 8+ and planning to go 9).

My problem is i log in with no problem but when i refresh main mail page it gives an error "unexpected error occured" and redirects me to login page.

I changed banner and background image. But i do not know if this problem was persist in default banner and background because after installation i directly changed the images.