Try:

/_layouts/15/new.aspx?ListTemplate=SpreadsheetImport

Sometimes, reducing the number of defaults in the URL can get around the hiccups.

[Edited]

Try:

{mysite}/_layouts/15/start.aspx#//{Sitecollection}/{mysite}/_layouts/15/new.aspx?ListTemplate=SpreadsheetImport&ListBaseType=0&DisplayName=Spreadsheet+Import

This should redirect you to:

{mysite}/_layouts/15/new.aspx?ListTemplate=SpreadsheetImport&ListBaseType=0&DisplayName=Spreadsheet+Import

The process has an interim page that you need to hit on the way to the url you listed. Also previously I had removed a few arguments from the URL. That, it turns out, is another way to generate the errors you are experiencing. All 3 arguments are required for it to work.

Try: /_layouts/15/new.aspx?ListTemplate=SpreadsheetImport

Sometimes, reducing the number of defaults in the URL can get around the hiccups.

Try:

/_layouts/15/new.aspx?ListTemplate=SpreadsheetImport

Sometimes, reducing the number of defaults in the URL can get around the hiccups.