Hello Luke,
I am a Microsoft user like you, providing solutions to community members; I am NOT a Microsoft employee.
The issue your guest users are facing when downloading files in the new Teams experience is likely due to permission limitations for guest access within Teams channels, here are some steps to troubleshoot and resolve the problem:
Step 1: Verify Permissions and Access Check Guest Permissions: Ensure that the guest users have the appropriate permissions in the Teams channel and the underlying SharePoint site where the files are stored. Go to the Teams channel settings and verify the permissions assigned to guest users. SharePoint Access: Check if the SharePoint site has any restrictions or special settings that might prevent guest users from downloading files. Navigate to the SharePoint site associated with the Teams channel and verify that guest users have access to the files.
Step 2: Clear Teams Cache Close Microsoft Teams completely. Press Win + R, type %appdata%\Microsoft\Teams, and press Enter. Delete all files and folders within this directory. Restart Teams and try accessing the files again.
Step 3: Use Classic Teams as a Temporary Solution Until the issue is resolved in the new Teams, instruct users to use the classic Teams client for accessing and downloading files.
Step 4: Check for Teams Updates Ensure that both the new Teams and classic Teams clients are updated to the latest version.
Step 5: Browser and Application Settings Clear the browser cache and cookies if users are trying to access files via a web browser. Ensure that the browser is up to date. Desktop Application Settings: Verify that the desktop applications for Word, Excel, etc., are updated to the latest version. Ensure that the Office applications are logged in with the correct credentials.
Step 6: Check for Azure AD Conditional Access Policies Conditional Access Policies: Verify if there are any Azure AD Conditional Access policies that might be affecting guest users' ability to access or download files. Coordinate with the IT admin of the organization hosting the Teams channel to check and modify policies if necessary.
Step 7: Report the Issue to Microsoft Support If the problem persists after performing the above steps, you should probably report the issue to Microsoft Support at https://support.microsoft.com/contactus/ or leave your feedback on the official MS TEAMS feedback website https://feedbackportal.microsoft.com/feedback/forum/ad198462-1c1c-ec11-b6e7-0022481f8472
Hope this helps
-Stephen N.