Outlook - Online mode address book search fails with "Operation failed" when Name only is ticked.

I hope you had this issue and managed to solve 😂 pulling my hair already.

Symptom: 

In classic Outlook for Windows, open the Address Book → set Search to Name only → type any single character → "The address book operation failed." Switch to More columns and the exact same search works fine. Ctrl+K resolution in the To: field works. OWA people search works. Mail flow is fine. It's only the Name-only ANR seek in the dialog.

Environment:

Exchange hybrid, mailbox in Exchange Online, dir-syncedfrom on-prem AD via Entra Connect

Classic Outlook for Windows, online mode (Cached Mode is not permitted here — this is the constraint, I can't just flip it)

Affects all users, all machines

What I've already ruled out:

Test Result

• Other address lists (Contacts, All Users) Same failure
• Ctrl+K in To: field Works
• Safe mode (outlook.exe /safe) Same failure — not add-ins
• Multiple accounts / multiple machines Same failure
• Brand-new mail profile Same failure — not profile corruption
• Addressing order (Tools > Options) GAL set first, not "Choose automatically"
• Off corporate network (5G) Same failure
• AddressBookPolicy / OfflineAddressBook on mailbox Both blank (inherit defaults)
• Get-OfflineAddressBook Default OAB, IsDefault: True, web dist enabled
• Get-GlobalAddressList Default GAL, IsDefaultGlobalAddressList: True, stock default RecipientFilter