When you are trying to install a solution from the Office 365 Admin Center, but you get a generic error and it is saying to try again later, this might be related to an issue with a Retrieve Multiple plugin that is registered for all entities.
You go to Office 365, then you go to Admin. After that in the left menu you expand Admin and click in Dynamics 365, it will open the Instance Picker page with all the instances that you have.
After that you select the instance and press the Solution option, it should open a new page where you can install/update/uninstall solutions to that CRM instance. If it isn’t opening and you are getting an error it might be the problem that I described before.
You need to open the solution and go to the SDK messages and check the message column and filter by the message RetrieveMultiple, or if you know the assembly, open the PluginRegistrationTool and disable the plugin and try to open the solutions page again in the Instance Picker page.
Hope this helps.