Latest OS update & related errors

Dear All,

Since the last update, several unexpected errors appeared on my iMac.

Firstly one unrelated to this update:

1. I have forgotten my security questions to change the Apple password. I've tried following the suggested actions to remedy this but - there is no direct support to Bosnia (where I am) and other than asking you, kind users, I can resort to no other option. How do I change the security questions, if there is a way to do that? Please help.

After the update, however, these issues appeared:

1. Constant appearance of a window with "enter your iCloud password", every time I turn on my Mac. This leads to "agree with terms and ..." and, on to "setting up your mac", which the OS does every single time I turn the Mac on.

2. Optimizing your Mac banner appearing in my upper right display corner.

3. Refusal to recognize my Transcend mobile drive as backup disk which, until update has taken effect, was working flawlessly.

How do I sort this out?

Very grateful in advance,

Ahmed