Jetpack & Wordfence
Jetpack or any other security plugin sometime causes the “something went wrong” error during import. “The woocommerce CSV importer breaks down a CSV in smaller parts and handles them one by one. This may lead to multiple requests in a short period of time. Security plugins sometimes block incoming requests if they reach some sort of threshold.
ERR_BLOCKED_BY_CLIENT 503 Service Unavailable
Something was blocking request. Jetpack was blocking them at a certain point in time. After disabling jetpack the import went smoothly!
The same happend with a recent version of Wordfence lately. See this forum thread for more information.
So beside disabling the plugins during import, this can also be solved by changing the setting “Number of products to process simultaneously”. If you set this to a higher number, the CSV file will be split by that amount. but be aware, do not put it to high else time-outs can appear.
Personally, i always start with a small file and process about 10 products at a time. If something goes wrong i only have to check the CSV file for those 10 products and fix it. Wrong formatted fields can lead to errors. Be sure to check out the knowledgdebase for all information about setup and formats.
More information about common error can be found here: https://allaerd.org/ajax-error-import/.