@koubel
Yes, all URL-s that I tried (outlook.office.com, outlook.office365.com and my work e-mail) are working now in mobile view. Error 500 is gone.

I have the same problem. My work e-mail stopped working from the main page, now it also stopped working from outlook.office.com and outlook.office365.com

It thinks Opera for Android is a legacy browser (Opera 92.1.4866.85656 1909216408, Android 14, Nokia X10).

UTC Date: 2025-10-16T09:34:04.662Z
Client Id: 909BFA61BE0A460FBE7C5FEBA0CA1B1E
Session Id: a3f5f9e1-32f8-4164-a820-a87e2dc14393
Client Version: 20250919.03
BootResult: fail
Back Filled Errors: None
err: Microsoft.Exchange.Clients.Owa2.Server.Core.OwaBasicUnsupportedException
esrc: StartupData
et: ServerError
estack: Error: 500
    at Object.D [as createStatusErrorMessage] (https://res.public.onecdn.static.microsoft/owamini/20250919.03/scripts/owa.miniindex.js:2:181517)
    at https://res.public.onecdn.static.microsoft/owamini/20250919.03/scripts/owa.miniindex.js:2:180078
st: 500
ehk: X-OWA-Error
efe: GV2PEPF00003836
emsg: ErrorOwaBasicUnsupportedLegacyBrowser

Last line says ErrorOwaBasicUnsupportedLegacyBrowser - unsupported legacy browser? The fault is on the Microsoft side then, they probably check the Opera version numbers and expect the desktop browser number which is higher, or something.

If anybody has a workaround, I would like to hear from you.