Don't know. Might have something to do with install leftovers in the temp directories or messed-up Opera autoupdate scheduled task or corrupted program files.
Delete all files and folders in "C:\Windows\temp" and "C:\Users\yourusername\AppData\local\temp".
Uninstall Opera through settings in Windows, and choose to keep your data (if you want).
Delete Opera's program files folder if it's still present. See the URL opera://about for the install path before you do all of this.
Load up the Windows Task Scheduler and delete any Opera entries that remain.
Then, download the offline Opera install from https://www.opera.com/download, launch it, click "options", check that all the settings are set to your likeing and install.
Then, see how things go from there. might be better with a more thorough uninstall/reinstall like that.