Some ax reports call into helper assemblies for various purposes. These assemblies are normally deployed along with the report. There are also some changes made to ssrs configuration when ax reporting extensions are installed.
If they are using a different server now, it could even be the case that ax reporting extensions have not been installed.
Given the error message that a helper assembly could not be found, I suspect the helper assemblies are either no longer present on the reporting server after the upgrade, or that the configuration changes to allow ax reporting extension to run got lost when the server was upgraded.
The helper assemblies should be in the reportingserver\bin folder. The full path will look something like this. C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services\ReportServer\bin
The folder MSRS10_50.MSSQLSERVER might have a different name on your machine. The name depends on which sql version is installed and what instance name was used for the report server instance.
If the helper assembly is missing from that directory, first make sure the setting for which ssrs instance to use is correct in Ax. You'll find the reporting server info under the menu command Tools -> Bi tools -> Report servers. Make sure it is pointing to the correct server instance. If it is correct, try redeploying the report from aot and see if that fixes it. That should ensure the assembly is present on the reporting server. But it won't fix any ssrs configuration problems.
If the report still doesn't work, then check the ssrs configuration settings. See the article here for what settings are supposed to have been added to ssrs configuration to enable ax reporting extensions: technet.microsoft.com/.../hh389762.aspx
Check if these settings still exist, and if not, add them.
Hope this helps,
Lance.