I submitted a bug to Esri a while ago (several months) and am still waiting on resolution to this issue. We have been passed around to several Technical Support personnel. I thought others here might have also encountered this issue in Portal and might have insight, or might find it helpful.
Overwriting a csv in Portal fails with an unknown error that also throws a error in the server log stating "The dataset has an invalid definition... Unknown field type in schema.ini. [...]" We have traced this down to an issue that arises when the following occurs: