Connecting to a iMap / Exchange SharedMailbox in Exchange is not possible

I want to add an Imap-Account to the client in Nextcloud. For a standard account it works fine.

But: I want to access an Exchange Shared Mail Account.
Our Exchange Administrator says, that the syntax for the IMAP User to access a shared mailbox is:

user@domain.com\sharedmailboxname

Within my Apple Mail Client or thunderbird it works fine. But it seems that Nexcloud Mail Client can not handle with this kind of username with the \ as a seperator to the sharedmailboxname. It neither works with / nor with \ in Nextcloud.

What to do? Is this a known issue? I could not find any information in the community.

Concerning the format for addressing a shared Mail-List, I found the following information in the Internet:

The IMAP syntax for a shared mailbox depends on the email provider, but common formats include
username/shared-mailbox@example.com or username|shared-mailbox@example.com, where you use your own login credentials but specify the shared mailbox in the username field. Some clients may also use shared-mailbox@example.com with your personal account’s credentials and access it via a different username structure or a separate “Shared Mailbox” setting.
Common IMAP syntaxes for shared mailboxes
your.username@example.com/shared-mailbox@example.com: This is a common format for platforms like Microsoft 365, where you use your personal email address and your personal password for authentication, and then specify the shared mailbox address in the username field.
yourusername|shared-mailboxusername: Some services, like GFI Archiver, use a pipe (|) as a separator in the username field. You would log in with your personal username and password, but the username would look something like john.doe|sales.
username/shared-mailbox: This format is often used for server-level configurations, especially when using a client that supports different authentication methods. For example, you might see authuser=user/shared.

Thanks in advance and best regards.

Did you activate the Oauth-App from Nextcloud App center?

No, but this is not neccessary for connecting to an email account.

As I’ve mentioned, the access to the mailserver with a user and password to connect for a single / normal account works fine. But the access to a shared mailbox while using the syntax username/shared-mailbox@example.com or username\shared-mailbox@example.com does not work.

Maybe the Nextcloud clients escapes the slash or backslash? Old maybe it just strips things after the username? Or just can’t handle with special characters?