Opera mini is a robot, since it downloads the page from the server and renders an altered version of the page which is sent to your device. Thus it lacks being able to login, retain cookies, etc.
From the linked Wikipedia page:
For this reason, Opera mini ...
Well, it appears that classic Nokia comes with Nokia Browser for Symbian as the default browser.
Stack Exchange sites are not supporting all existing browsers. List of supported (and explicitly unsupported) browsers can be found here: Which browsers are officially supported, and what else do I need?
Try installing standard mobile browser like Chrome or ...
According to Which browsers are officially supported, and what else do I need?:
Stack Overflow appears to function properly in 12.0.1
Explicitly unsupported: Opera Mini (source)
And from Unable to start a bounty using Opera Mini:
Unfortunately at the moment we don't support Opera mini.
Reason: a combination of amount of ...