When accessing the display of a document library, you get this error message:
An error occurred during the processing of
/site/SharedDocuments/Forms/AllItems.aspx. Code blocks are not allowed in this
The view (usually AllItems.aspx) is corrupted. Since I know the users do not open the site with SharePoint Designer, how it got corrupted is "just through the magic of Microsoft".
Open the site through SharePoint Designer. Find the document library, expand the Forms folder and find the offending view. It will be an aspx page and will have a blue exclamation point next to it, indicating that it has been customized from the site definition. Right click on it and choose "Reset to Site Definition".
It will now work normally.