The error message was specific: “Cannot start Microsoft Outlook. Cannot open Outlook window. The set of folders cannot be opened. An unexpected error has occurred.” That phrasing points in a few possible directions — a corrupted data file, a bad add-in, a broken installation — so the first order of business was to protect what we had. Before touching anything, we made a backup copy of her Outlook data files.
The next step was launching Outlook in Safe Mode, which strips away add-ins and loads with a minimal configuration. In Safe Mode, we created a new Outlook profile using her login credentials. Outlook loaded up, connected, and began syncing her messages. A quick comparison against the online version confirmed everything was coming through correctly. So far, so good.
Since add-ins are a common source of Outlook problems, we disabled all of them while still in Safe Mode, then quit the program and tried launching normally. No luck — the same error came right back. That told us something useful: add-ins weren’t the culprit.
We ran the Microsoft Office Online Repair tool, which does a more thorough job than the quick repair option — it checks Office installation files against Microsoft’s servers and replaces anything that’s out of place. This is Office 2021, and the repair process ran through its full check. After it finished, we launched Outlook again. Same error.
At that point, we located the OST and PST data files, created backups, and renamed the originals so Outlook couldn’t load them. Then we went back into Safe Mode and deleted the old profile that had been there before we created the new one. That was the turning point.
With the old profile gone, Outlook Classic opened without any issues. We re-enabled all the add-ins, and it still worked. We sent a test email, replied to it, and confirmed everything was functioning normally. To round things out, we ran a Windows 11 update and updated the HP BIOS, then did a full reboot. Everything held.
Outlook profile corruption doesn’t always announce itself clearly. The error message here — “the set of folders cannot be opened” — sounds like a data file problem, and that’s a reasonable first guess. But in this case, the profile itself was the issue. The data files were fine. The installation was fine. It was the profile record tying everything together that had gone bad, and once that was cleared out and rebuilt, the problem disappeared.
If Outlook is refusing to open on your PC and you’re not sure where to start, bring it in. We’ll work through it methodically until we find the root cause.