Outlook 365 Pro Shared Mailbox weird behaviour

I usually don't encounter impossible to solve issues, but I am completely out of ideas here. So here's the problem:

I want to open a shared mailbox via Outlook 365 Pro-Plus.
If I try it with: File>Accountsettings>New
It automatically grabs the emailadress from somewhere... pre-filled field (which is wrong)
which would be fine, if it worked!

However Outlook is just giving me the finger by repeatedly asking for the passwort, which, since it's a shared mailbox, is expected and we have to "bypass" by clicking on "sign in with another account".

This is the expected behaviour, but instead of letting us type in our own password, it already grabs a password from somewere and then just asks for a password again..and again.. Sometimes the loop stops and says it's connecting, but then just gives us a 0x80040115 Error.

ODDLY ENOUGH:
Opening the shared mailbox via Webaccess https://portal.office.com works just fine.

Now here are the things I've already tried:

1. deleted all OST-files associated with the shared-mailbox, then restarted outlook, so it would reinitiate the OST's
2. Deleted all cached credentials in Windows, then reapplied step 1
3. reinstalled Outlook 365, after deleting all cached credentials
4. reinstalled O365, after cached credentials wipe and deleting OST's
5. made sure everything's set up properly on our side (on-prem AD)
6. made sure everything's configured right in O365 via admin-panel
   
   

OS: Windows 10 Enterprise 1903
Outlook: Outlook 365 Pro-Plus

I haven't looked into the event viewer, yet.

Desired outlook behaviour:

A) blank field when opening/adding another mailbox
B) outlook letting me type in a password myself.

Any ideas or even solutions would be greatly appreciated! :)

Some more aspects regarding out O365 setup:
-there are about 7 co-workers, for which it works fine, as it should.
-config is EXACTLY the same for each user
-shared-mailbox permissions are set via powershell, using an userarray and a for-each function.

which is basically this:

$automapping = $false
$users = @()
$users += "user@contoso.com"   
$users += "user1@contoso.com"

foreach ($user in $users)
{
    Write-Host "Adding mailbox permission in " $mailbox " for $user " with automapping:" $automapping " 

    Add-MailboxPermission -Identity $mailbox -AccessRights FullAccess -AutoMapping $automapping -User $user
}