The error you're seeing almost certainly has nothing to do with the error Joost saw. It's a general programming error that means the program tried to do something that is not allowed in its framework, so it can happen almost anywhere there is a bug of that type.
In your case, I'd condition-out or otherwise remove the UEstyles.css just to see if it would build properly afterwards. If it does, you know the problem is in that file. There is something that Flare doesn't like in there, and you have to try to figure out what. In a situation like this, I typically do things like trying out the CSS in a sample project, open it in a text editor to see if there are any obvious problems (like junk data that might suggest corruption, or some malformed XML). Then either contact support or submit a bug report.
Flare v6.1 | Capture 4.0.0