We have had a few cases where an older version of Outlook was updated to Outlook 2019 or later to accommodate modern authentication. It would work for a while then the user would begin getting repeatedly prompted for their password because it stopped using modern authentication.

For some reason, the following registry entries were either missing or changed. Changing/adding them resolved the issue:

HKCU\SOFTWARE\Microsoft\Office\xx\Common\Identity\EnableADAL
REG_DWORD    1

HKCU\SOFTWARE\Microsoft\Office\xx\Common\Identity\Version
REG_DWORD    1

HKCU\SOFTWARE\Microsoft\Office\xx\Common\Identity\DisableADALatopWAMOverride
REG_DWORD    1

HKCU\SOFTWARE\Microsoft\Office\xx\Common\Identity\DisableAADWAM
REG_DWORD    1

HKCU\Software\Microsoft\Exchange\AlwaysUseMSOAuthForAutoDiscover
REG_DWORD    1