Background:
We recently ran into this issue when attempting to access our site through the Content and Structure page on the main site. This error presented itself when attempting to expand the tree for a particular site. After clicking the plus sign and waiting for a while, the error message would appear just below the site name. After receiving the error, attempting to expand any other site resulted in the immediate display of the message. Also, when trying to access the Content and Structure page from any site under the problem site, SharePoint would display the Error page with the very useful “An unknown error has occurred” message. This was a big problem for us because we needed to move items, lists, and sites.
Solution:
Ultimately the problem was caused by the server timing out on the request. Our system administrator changed the IIS timeout settings from 120 to 600 for an unrelated issue. After the setting was changed we no longer encountered the error. We made the change on a development server that was having the same issue, and this solved the problem there as well.