If all of these different IMAP accounts are hosted on the same server, then you may be only allowed to have so many IMAP connections at once, controlled on the server-side. Outlook uses more than one connection to keep each IMAP account up-to-date when active.
So it'd be up to your email provider to check/confirm/fix this.
From Google Gmail help:
Too many simultaneous connections
that if you have multiple users frequently accessing the same account
or more than one IMAP1 client accessing Gmail at the same time, you
may reach a connection threshold and receive the error message, 'Too
many simultaneous connections.'
Gmail currently has a limit of
10 simultaneous IMAP connections per account.
Please note that
your mail client may actually open multiple connections in the
background. This means it's possible to reach a connection threshold
simply by using only two mail clients to access the same account at
the same time.
If you often use multiple mail clients or mobile
devices with the same Gmail account, we recommend closing and/or
signing out of any mail clients you're not actively using.