Multilingual multisite problem after changing standard language
Here is a hint (as in your question), that might help: make sure the URL of the homepage is defined as a "multilingual" variable. The relative URL to display as the front page is specified within admin/config/system/site-information.

If you want to be able to specify a value for it that is different for each of the site's languages, make sure you have made it a "multilingual" variable (it should say "This is a multilingual variable." in the description of that variable on that page).

For more details about how to get this done, refer to my answer to "How to make content translation available for front page?".