The way attributes worked in v8 changed from how they work in 5.7. There is backward compatibility code in the core, but it is not 100% backward compatible. ProBlog (and siblings), make heavy use of attributes.

I had to make extensive changes to ProBlog to get it to work with version 8.2.1, including overriding the built in select attribute type. Problog does some funkiness in the way it extends that attribute with a hidden, uninstalled attribute used simply to have a class to handle filtering. I'd say that it's just simply incompatible, and if I wasn't working on a site that already had it handling lots of content, I would have looked for a different solution.

You probably won't get an answer from support, since it's considered end-of-life and is no longer for sale in the marketplace. Probably best not to use it on any new sites that aren't already using Problog.

Would you be willing to share what you did to get it to work? We have a client site we're trying to upgrade and have the same issue--can't move off ProBlog just yet but want to upgrade C5.

Would be happy to use your code with no support if you are happy to share :) please