Outlook 365 cannot open OST for second account

Windows 10, Office 365

I want to email merge using a second group account, set as default for sending. The problem is the emails are stuck in the outbox because of Error 0x8004010F: Outlook data file cannot be accessed.

Main Account: Me, which has an OST.

I have connected to the Second account using the usual "Add more mailboxes" and can see all emails inside it.

Second Account: Group mailbox (described as an Exchange account in Account Settings), set to default, and has an OST file, but errors associated with it.

I can set it as the default Email account but If I try to set it as the default data file the following: - cannot be the default message store - cannot be opened and to verify my default store

Not being able to open this second email account's OST file looks like why I cannot send as that account name. Any suggestions?