The provided code behind and configuration seems to be correct. Thus, it would be best if you could provide us with small runnable sample project exhibiting this behavior, so we can review it locally and advise accordingly.
Additionally, you can try to hardcode the full connection string in the TRDX file instead of using a named connection string and test if the report can be displayed in the viewer with such configuration.
Note that the Telerik Reporting 18.104.22.1688 version had an issue with empty design time values for data source parameters, so we recommend you to upgrade to the latest internal build where this issue is fixed and verify if the problematic behavior persists. Please, download the latest internal build
from your account and give it a try.
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