I struggled with this issue for too long before finally tracking down how to avoid/fix it. It seems like something that should be on StackOverflow for the benefit of others.
I had an SSRS report where the query worked fine and displayed the string results I expected. However, when I tried to add that field to the report, it kept showing “ERROR#”. I was eventually able to find a little bit more info:
The Value expression used in [textbox] returned a data type that is not valid.
But, I knew my data was valid.
Found the answer here.
Basically, it’s a problem with caching and you need to delete the “.data” file that is created in the same directory as your report. Some also suggested copying the query/report to a new report, but that appears to be the hard way to achieve the same thing. I deleted the .data file for the report I was having trouble with and it immediately started working as-expected.