This is what I found on a search.. its the + signs in the search that are messing things up for me.
Allow + signs in URL when using IIS 7.x – Double escape sequences
08
Jan
Just had to migrate a SharePoint site collection from a MOSS 2007 to a SharePoint 2010 environment. Migration has been done with backup and restore of the site collection. Site collection has been updated in the new environment and has been migrated successful but after migration server gave me an error from IIS for the site collection telling me ‘HTTP Error 404.11 – Not Found` and ‘The request filtering module is configured to deny a request that contains a double escape sequence’, so what happened?
I checked the site collection in the old environment and with this the URL. Within the URL a ‘+’ character was included. This seems to be working on IIS 6.0 and will bring up an error on IIS 7 as double escape sequences are not allowed by default as it can be a security issue.
Normally you need to screen the site which needs to be migrated and need to ensure that there are no + signs anymore but when the customer does not allow any changes in structure or even names within the site collection a workaround is needed :-/. To do this you need to set down the security for the webserver but with this you can easily allow the blocked sings which will cause the ‘double escape sequence error’.
If you really require the use of the “+” character in the URL path you can disable the validation by setting the ‘allowDoubleEscaping’ attribute in the web.config of the WebApplication within the IIS e.g. the SharePoint is running in. Open the web.config and look for the following section: