Hi Otheus,
Sorry for not responding before. I'm the main author of Data Transfer, as you probably know. I haven't heard of either of those problems happening before, I don't think: either the 500 error or that PHP notice. I'm guessing that the two are related, and my further guess is that the problem is somehow due to unclosed curly brackets in one or more of the pages - i.e., there is a "{{" that has no corresponding "}}". Which should not crash the extension, of course, but maybe that's what is happening.
Have you tried Special:ViewXML for other categories or namespaces on your wiki, and if so, do those work?
-Yaron