This error happens when the data entered for a Participant Attribute is longer than 255 characters. 

To resolve, you may enter the transaction manually into The Raiser's Edge, and then delete the transaction from the plugin.

Alternatively, Support can assist with editing the raw XML of the data to shorten it to under 255 characters. If you want to pursue this: Click Chat with Support and reference this article. 
Note: if you are bulk processing, you need to first determine the transaction which is prompting the error, by process of elimination, prior to reaching out to support.