Zephyr Test import - error uploading the file

Hi, we are getting an error uploading test cases into Zephyr using the internal import function.

This has worked previously but is now giving an error:

An embedded page at prod.play.zephyr4jiracloud.com says

error uploading the file.

it is not progressing to the mapping of columns, just rejecting the excel file. 

Is this a permission or firewall restriction?