Lately in light of some changes in 500px I became PRO from current awesome. following this the export to 500px with plugin 1.10.0 became impossible. I believe this has to do with 500px API rather than LR, if somebody knows a workaround, kindly share it.
HI, I have the same issue and I reported it to 500px. They have acknowledged this as an issue and are working on it. In the meantime I just export photos from LR to a desktop folder ( they contain all the meta data that the 500px publish tool holds e.g Title, 3 keywords, license e.t.c.). Then I use the upload tool directly from the 500px website ( under photo management ) to get my new photos on board. A longer process but it works until they fix it.
My symptoms are a little different. Most existing photos that have been updated (at least LR thinks so) can be published, but new photos in the same collection can't be. I didn't get an error about missing keywords or titles, but I addressed those problems without much difference. I also tried unchecking the option to publish to Fresh, but no difference there.