I am using free DeepL and running the latest version, though the issue started happening with the previous one.
Were you able to translate at all in the previous version before it started happening?
If no translations at all are coming through, I would bet that your 500.000 characters on the free limit are maxed out for this month. If that's the case, the error message itself is a bug; it should give another message back.
If they are not maxed out, let me know, I'll be able to release a version tomorrow that makes it possible to disable batch requests for DeepL and we'll see if that works for you.
Great!
If you would like to keep using API's that can produce better quality translations, I would suggest following the guides in either https://itch.io/t/5352880/free-llm-api-resources-api-setup-guide or https://itch.io/t/5344129/feedback-using-gpt-api-gametranslate-for-switch-vn-tra...
Let me know if you struggle to get any of them properly setup if you do use it. :)