Resolved
Langdock for Excel and Langdock for Outlook can be installed again from within the desktop or web Excel and Outlook clients.
Investigating
Langdock for Excel and Langdock for Outlook are currently unavailable for new installations. Existing installations continue to work as expected. The issue also appears to affect other plugins in the Microsoft Marketplace.
We're investigating the issue and monitoring Microsoft's status page for updates: https://status.cloud.microsoft/m365