I have also been having this issue, and have emailed them. They gave a few suggestions (your standard try a different browser, clear cache/cookies, etc), and I had already deleted and re-downloaded google chrome. I thought it had begun working again, but have not had to import anything else in a few days (& has seemed to just work every 5th or 6th time I try since the error began showing up), & consistently started having errors again today. In the browser error console it is saying the “server is responding with a status of 400”, most people online suggest clearing cookies/caches, deactivating extensions, etc and those things have seemed to make no difference.
I emailed them again today, and can update you on if there is any other suggestions/updates/things to make it work!