The "Opera Mini 7.5.1" you are searching for is an extremely old version of the mobile browser originally released over a decade ago. While it still surfaces on third-party APK platforms, attempting to run this legacy application on modern Android devices poses severe security and functionality risks.
Fix: Go to Settings > Downloads > Clear download history. Then restart the browser. Version 751 includes a fix for this, but legacy settings may linger. opera mini 751 on androidapk new