I’ve ran into this wee SSRS issue today, a quick fix mind you, but I thought its worth mentioning.
The issue is in-relation to the error message: Deserialization failed: The ‘DataType’ attribute is not declared. Line X, position Y.
This issue surfaces when migrating SSRS reports to a newer version, in my case it was from SQL Server 2008 to SQL Server 2008 R2, although it has been noted when migrating to other versions of SSRS. Also, interestingly enough, if you were building the SSRS reports from scratch (i.e. using the XML schema to build the report), then you will run into the same issue if you used the DataType attribute on your parameters (even though the DataType attribute is a valid XML element as defined by the SSRS report schema).
Anyway the error generally happens on your report parameters, specifically on Integer DataTypes (either as part of the Available Values or Default Values of a parameter). The way to solve this issue is by editing the SSRS .RDL file manually yourself, either through visual studio using the Edit Code button which comes up with the issue (picture below) or through your favourite XML editor.
If you go to the line number where the error has indicated, you will find an XML that looks like this:
All you really need to do is remove the DataType=”Integer” attribute from the Value tag, and everything should work fine. I recommend rebuilding the report after you have done that just to make sure everything went down well.
Hope this helps!