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First of all, the message is not an error, it is a warning. The warning means that there is no distinct order of events in the Activity table, although you have defined a sorting column. This happens when two events of the same case have the same timestamp and the same sorting value. The sorting column is used as the secondary sorting criteria, which means that the Activity table is first sorted by timestamp, and in case of equal timestamp, it is sorted by the sorting column.
Therefore, you should check your source table. Otherwise the data model is still successfully loaded, however, when the sorting is not defined correctly, the numbers in your analysis might change after a data model reload (because the order of the Activities might change then, which for example produces different variants).