Plugin Update and Install partially broken

Starting a few days ago, when I update a plugin, the page displays the “updating plugin” and whirly message, but never goes beyond that point. When I try to install a new plugin, I get the message that the plugin is being downloaded but nothing further.

However, in each case the update seems to have been applied or the plugin installed. Maybe something is not being cleaned up after update or installation?

How can I fix this?

Thanks!