I have a client who has two shared mailboxes in Outlook and they started to receive a password prompt asking for the password of the shared mailbox account. Typically the shared mailbox accounts do not have password. I know that it is possible to assign a password and then grant webmail access but in this case, no matter what password we tried, the password prompt never went away.

It appears that if you have previous Outlook profiles with that particular account added to said profile and it was not an actual Exchange or Exchange shared mailbox at one point then you will receive a password prompt as Windows is trying to index that file for search.

Source: https://gcits.com/knowledge-base/remove-microsoft-windows-search-protocol-host-prompt/