When I log in to the phone (and only when I login) the screen flickers, flashing with the blue desktop background alternating with the normal black login colour. Once I have input my pin numbers and the phone has accepted them it is perfectly normal. My daughter has the same problem with her FB2 also. Both our phones have been doing this for months now. My phone is always on full brightness while my daughters is on roughly half. Any suggestions?
I think what you are describing is a known bug:
I wouldn’t worry much about because it anyway disappears very soon after the start. Let’s hope this bug will be fixed sooner or later.
If you have further questions please continue in the topic linked above.