Using older versions of Outlook for Mac in business environments
I'm too lazy to review the previous 20 pages... and forget if it happens in both Yahoo added as a Microsoft cloud account and as IMAP... but if you have it added as a cloud account, try IMAP and if imap, try cloud.
cCoud accounts sync calendar & contacts from the server to Microsoft servers then add the account using an Exchange-like protocol and is supported for Gmail, yahoo, and iCloud accounts.
To check the account type: tools > accounts > select account. Do you see the IMAP server names? If not, its a cloud account.
To use Microsoft cloud accounts:
Enter the address and let Outlook find it and set it up.
To use imap:
After Outlook finds the account, click Not Yahoo in the upper right corner then choose IMAP on the next page. If you see a slider for Microsoft cloud, slide it off (its above the account type buttons and may or may not be present.)