I had this problem yesterday and fixed it this morning. For me, my variables were defined from a collection, and a change in my data source (SharePoint) made my collection formula incorrect, causing my variables to throw this error.
My code looked like this:
ClearCollect(colCollection, library1);
Collect(colCollection, library2);
Collect(colCollection, library3)
This collection formed the items property of my gallery and several variables were defined from it. library1 and library2 have a SharePoint Lookup column that they share. When I received the error message "the schema for your data sources has been updated, which have reset the following collections and variables" it made me think that the problem was with the data, not the app. I pulled the two libraries up side by side and I found a difference between them: one had been checked for "allow multiple values" and one was not checked. This is how the "data sources" had been updated. Either I, or a colleague, accidentally checked that box, which prevented colCollection from being formed, which prevented my variables from being defined.
In my case, the problem related back to my data source. The app worked fine, the data source changed, then the error occurred. Hope this helps someone's troubleshooting in the future.
