Hi Morten, I'm Joshua and I am happy to help.
Sorry to hear you're experiencing this issue!
It sounds like you are experiencing an issue with syncing a Shared Teams Channel to OneDrive when the files belong to a different tenant. Based on your description, it appears that the B2B setup is working correctly, and users from the other tenant are able to access the files and folders in the Shared Channel through Teams and SharePoint. However, when attempting to sync the files to OneDrive, you are receiving an error message indicating that the folder may not have the correct permissions or may belong to a different organization.
This issue is likely related to the fact that the files belong to a different tenant, and OneDrive is not able to properly access or sync with them. As you mentioned, you have tested a variety of settings to try and resolve the issue, but have not been successful. Unfortunately, based on my knowledge cutoff, I couldn't find any official documentation about the issue of syncing Shared Teams Channel with OneDrive when files belong to different tenant.
Let me know if you need further assistance.
Regards,
Joshua.