I use Evolution with the usual GTK treeview of folders on the left. I noticed that some times it is not possible to expand or collapse folders that include other folders in the treeview.

After some investigations I discovered this is always triggered after I click on the "new mail" icon that appears on the notification when new mail arrives. This is always reproducible.

It is however possible to expand folders if one drags a mail into them, but then it is still not possible to collapse them.

There is no other workaround that restarting evolution.