I don’t see anything wrong in your screenshot; the attribute name in your device config appears to match the name in your state report. My best guess is that the length of the string you’re reporting is too long. The maximum length is 65,535 characters for a string attribute. Are you bumping up against that limit with that state report?
My second guess is that the state report is actually succeeding but the error message is incorrectly getting displayed. After reporting state and seeing the error message, can you then visit the “Debug” tab and see if the state report was recorded in the “Recent Device States” panel?
If neither of those are the issue, can you please copy the content of the device log output for the invalid state report and paste it here so we can take a closer look? (Make sure there isn’t any sensitive information in the string before doing so.)