Hello @kshibata,
I have logged in and investigated your issue. Upon saving the Loco Translation, I encountered the same error message and hen I checked the console, I got this:
POST https://...../wp-admin/admin-ajax.php net::ERR_CONNECTION_CLOSED
In most case, your hosting provider may have a security rule that was limiting the access to admin-ajax.php. Some security or firewall software on the server (like mod_security) is configured too strictly. For example, it might not like that there are so many URLs in the HTTP request to save the table, and it might (erroneously) think that this is a spamming attack. Then, it blocks the save request and returns the error.
To fix this, you will have to contact your hosting provider. They should be able to find out more about this error in their server log files. Then, they can relax the configuration of the software.
Please let us know how it goes.