Hi Thorben,
Before moving on, I want to confirm with you first.
1. Did you create room mailbox for each meeting room in Office 365?
If you have created room mailboxes for the meeting rooms in the organization, by default, the allow conflict option is not checked. This means the room mailbox can decline the conflict by default. Meanwhile, please check the booking requests for the room mailbox is automatically accept or decline or select delegates to accept or decline.
I have a test about the meeting conflict in Outlook Web Access (OWA), select the room mailbox as the location when sending out a meeting request. My test room mailbox (Room.No1) uses automatically accept or decline booking requests. The screen shot below is my test result.
If you want to configure more setting about the room mailbox, please access the room mailbox and go to Options>see all options>Setting.
Note: You may need the full access assigned for the room mailbox to open it.
Reference
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Create a New Room Mailbox
@Alexandru, thanks for your information.
Thanks,
Eric Sun