Fix: Something Went Wrong. Your Mailbox Isn’t Available Right Now. – Office 365
While trying to access email in your web browser using Outlook Web Access (OWA), you may receive an error message similar to below.
Something Went Wrong
Your mailbox isn’t available right now. Please wait 30 seconds and then try to access it again.
Refresh the page
Most likely, your exchange administrator has made a change to your exchange license for your mailbox.
Toggling on/off settings such as your Office 365 license in the exchange portal can cause an interruption to your Outlook Web Access. The administrator may need to toggle on/off your license again for you to gain access back to OWA. If the exchange administrator has not made any other changes, this is likely the cause of your error.
This is the worst answer to this issue I've ever seen. What does "toggling" a license in a modern web-based app have to do with anything? Allow me to enlighten you: This error is usually seen when converting a shared mailbox to a newly added license to make the previously "shared" mailbox into a named user account that can be accessed remotely. As anyone knows, Shared mailboxes cannot be directly logged into, only via an authorized named user account. Once the conversion takes place, it takes a while to actually make the mailbox accessible.