@Julien bastin Thanks for posting in our Q&A. From your description, I know that we am migrating devices from other MDM to Intune and getting error messages when we try to deploy M365 apps. If there is any misunderstanding, please feel free to let us know.
For the error message "an update could not be installed because Office applications are open. (0x426E) ", Could you please end all the office process on the affected machine in task manager to see if it can installed? Based on the official article, it mentioned that "If any Office apps are open when Intune installs the app suite, the installation might fail,". We can see more details in the following link:
https://learn.microsoft.com/en-us/mem/intune/apps/apps-add-office365#before-you-start
For other error messages, we can refer to the following article to troubleshoot the app deployment issue:
https://learn.microsoft.com/en-us/troubleshoot/mem/intune/troubleshoot-app-install
If the issue still exist, It is better to create an online support ticket to handle this issue more effectively. It is free. Here is the online support link and hope it helpful.
https://learn.microsoft.com/en-us/mem/intune/fundamentals/get-support
Hope this issue will be solved as soon as possible.
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