Error "Data at the root level is invalid" when opening a report

Recently users reported that they receive an error when opening a particular report. The error is

"Data at the root level is invalid. Line 1, position 1."

The call stack of the error is

Even opening the report in Visual Studio was throwing the same error as shown below.

We exported the report from the AOT and analysed it ina text editor. It turned out that there was a CU/KB applied and the report was part of it. However the report was never upgrade and it had comments from the code upgrade tool. Fixing these comments in text editor, re-importing the report in to AX and deploying it fixed the issue.

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