I am trying to copy a site collection from one tenant to another without using any third party tool.
I am trying to do it with "save site as template" feature in SharePoint Online. But, when I try to save the site as template, unexpted error occurred pops up, but saves the wsp file in the solutions gallery, however I can not activate the wsp as there is another error.
Try create new empty Site Collection and create template from it. There can be time out while creating template, based on amount of data size on this site. (threshold of template size). If the error will appear also on empty site, create Microsoft Case and they will help you to resolve this problem.
There are some information and instructions about site templates. Note:To include the content of the site in the site template, select the Include Content check box. The limit is 50 MB and cannot be changed.
But if you want to migrate sites to another tenant I strongly recommend use migration applications, because there are lots problems with permissions, links, etc...
