The Submit Success Event event of the Web Forms for Marketers module is not migrated properly with the xDB Data Migration Tool 2.0.0 and xDB Data Migration Tool 2.0.1, which causes aggregation errors in the Sitecore log:
ERROR Aggregation Error Exception: System.ArgumentNullException Message: Value cannot be null. Parameter name: s Source: mscorlib at System.IO.StringReader..ctor(String s) at Sitecore.WFFM.Analytics.Aggregation.Processors.FormSummary.FormSummaryProcessor.OnProcess(InteractionAggregationPipelineArgs args) at Sitecore.Analytics.Aggregation.Pipeline.InteractionAggregationPipelineProcessor.Process(InteractionAggregationPipelineArgs args)
To resolve the issue, download and install the hotfix corresponding to your xDB Data Migration Tool version:
See the Readme.txt file inside the archive for installation details.