Hi,
When you save a Datalog file, you save a cluster with a user define elements on it. When you read the file back you must specify the type of data that you expect to read from the file. When you specify in the read a cluster different from what LabVIEW finds in the file you get this error.
It is possible that you attempted to open a file that was create with an older version of your app where the type actually changed.
I hope this helps. Let me know if you have any further questions.
Regards,
Juan Carlos
N.I.