If this is the case, you can view the errors by loading the page in Chrome, hit the F12 key and then click the Console tab. Often the the plugin causing the error can be identified by the error of the file path of the file causing the error.
If you can not identify the plugin, temporarily deactivate all plugins except Connection and the CSV Import extension and then do the import.
If the error still persists, then the theme is actually the cause which means you’ll need to temporarily switch back to the default theme.
If you want you can give me a temp admin account and I can logged in and look to see what the issue is exactly.