There can be several reasons associated with this issue. One common reason can be related to the virtual interface configuration of the controller. In order to resolve this problem, remove the virtual interface and then re-generate it with this command:
WLC>config interface address virtual 126.96.36.199
Then, reboot the controller. After the controller is rebooted, re-generate the webauth certificate locally on the controller with this command:
WLC>config certificate generate webauth
In the output of this command, you should see this message: Web Authentication certificate has been generated.
Now, you should be able to access the secure web mode of the controller upon reboot.