@talion1239 If the nothing works try these three troubleshooting tips

Clean Browser Data: Clear cache, cookies, and browsing history to eliminate potential data-related conflicts.

Test in Safe Mode: Run Opera in Safe Mode to determine if the issue is caused by extensions or settings.

Check Hardware Drivers: Ensure all your hardware drivers are up to date, as outdated drivers can lead to system instabilities.

I think the easiest and fastest way is to buy a third party app like Magus EML to MBOX converter. It is easy to use and very low in price so it can be perfect fit for any one who wants to export multiple .eml files to mbox in bulk.