Cannot access Outlook email: set of folders cannot be opened

Outlook 2016 starts, asks for my password, then throws Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened.

Webmail works fine so the account itself is alive. This is a POP account with a local PST that goes back years. What do I check first?

That error nearly always means one of three things: a stuck navigation pane, a broken profile or a damaged PST. Work through them in that order because the first two take a minute each.

1

Press Win+R and run outlook.exe /resetnavpane. This clears a corrupt navigation pane cache, the most common cause after an unclean shutdown.

2

If that fails, run outlook.exe /safe. If Outlook opens in safe mode, an add-in is the problem. Disable them under File, Options, Add-ins, COM Add-ins, Go.

3

Still failing, build a fresh profile: Control Panel, Mail (Microsoft Outlook), Show Profiles, Add. Point it at the same PST and set the new profile as default.

4

If the new profile also chokes on the PST, the file is damaged. Run SCANPST.EXE from C:\Program Files\Microsoft Office\root\Office16, browse to the PST and let it repair. Back the PST up first, the tool modifies it in place.

Years old POP PSTs are often ANSI format with hidden damage that scanpst only partly fixes. If scanpst loops or the repaired file still misbehaves, a dedicated repair tool like SysTools Outlook Recovery reads the file directly and exports clean mail to a new PST.

The resetnavpane switch did it. Outlook had crashed during Windows Update the night before which fits your explanation. Thanks Edwin.