Opera (from Version 9.0): How should the browser be configured so that there are no problems?

Activate SSL connection (Secure Sockets Layer):

Menu item: Options/Settings
Tab: "Advanced"
Category: "Security", Button: "Security Protocols"
Activate "SSL v2", "SSL v3", "TLS 1" and "TLS 1.1"

Trustworthy sites:

Menu item: Options/Settings
Tab: "Advanced", Category: "Security", Button: "Trustworthy Sites"
Select and add 

JavaScript:

Menu item: Options/Settings
Tab: "Advanced"
Category: "Content"
Select the control box "Activate JavaScript"

Password protected sites:

Menu item: Options/Settings
"Privacy": Deactivate "Use Cookies to trace password protected sites".

Allow pop-ups:

Menu item: Options/Settings
Tab: "General"
Dialogue fields pop-ups: Select "Open all pop-ups" or "Block unwanted pop-ups".