Likely a new GUI bug. There were similar issues before with 5.4 like interface view keeps "circling". When it happens only thing we can do is to wait for a bug fix version. Please open a case with TAC. Probably somebody already reported the same problem though.
I have the same problem here with the Fortigate 500E (image version 5.6.6). I have tried to update the image to 5.6.7 und rebooted the Fortigate, but noting better. Could you please tell me, how did you fixed your problem?
We had the same problem but in our case it was tied to a flag under a certain ADMIN PROFILE Edit Administration Profile "Name" Secure profile configuration flag on Web Filter in Read-Write that was the only one in "None" under that menu.