Today I was working on the report and the requirement was to add one more field in the existing table and show it. I have modified the data source accordingly and mapped the field in the table of SSRS report. When I tried to preview the same, it was showing the value as #Error.
I was little surprised and checked the mapping and data source. Everything seems to be correct and somehow the report is not showing the fields value in preview mode.
After some search came to know that it seems to a bug/by design in SSRS. For every report, you will see a .data file in your report project and it is basically caching the data. Just deleting that file and running the report again fixed the issue.
You can also just do refresh in the preview by the clicking on the button highlighted below.
Hope this helps.