Fixed. The problem was a missing or corrupted picture on a card.
Notice my error has to do with a PNG file.
Max retries exceeded with url: /wp-content/uploads/imgs/1/c/1cb836c0-s.png
You can try 3 things to get it working.
In Anki, go to Tools > Check Media > Tag missing. Then go to the card browser and select "missing-media" from the sidebar under Tags. Check and delete missing pictures.
This wasn't enough for me. I had to do the following to check the pictures of the cards that were created after the last successfully uploaded card in Migaku Memory.
Cancel the import on study.migaku.io after you get the error.
Open the deck, check the top card that was able to be uploaded, and remember its term
In Anki, go to the card browser, select the deck you are trying to upload on the sidebar
Order the cards by Created Date and search for the top term (step 2) in the search bar
Erase the term, and you should see that word in the browser with other cards that were created around the same time.
Going from Create Date, look at the contents of the cards created after the term
Check the Screenshot or Image fields for missing or corrupted images, and delete them
Sync and try the import again in Migaku Memory
If that doesn't work, check your Anki media folder for images created at around the same time as the top card and check for a small file size like 1kb, and delete it if it's empty. I had one of those and the picture was a tiny dot.