Unable to upload pictures to my library

Hi there !

I encounter an issue with my MailPoet installation (WP v4.7.1, MailPoet v2.7.6) :
When I want to upload an image, the popup get opened. Usually thsi popup allows my to select the image I want to insert in my newsletter. Instead of that, the popup shows me an iframe which contains the screen that invites me to upgrade my database (but the message informs me that the database does not need any upgrade).
If I click on “Continue to [MyWebsiteName]”, the iframe is redirected to the homepage of the website

Here is what happens :
1) I click on “Add pictures” (“Ajouter des images” in French)
2) An ajax request starts to : /wp-admin/admin.php?page=wysija_campaigns&action=medias&tab=special_new_wordp_upload&emailId=78
3) A second HTTP requests starts automatically to : /wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2Fadmin.php%3Fpage%3Dwysija_campaigns%26action%3Dmedias%26tab%3Dspecial_new_wordp_upload%26emailId%3D78
4) The content of this last request is shown in an iframe

The same thing happens when I click on “Télécharger encore plus de thèmes” (“Download more themes”).

Do you know how I could fix it ?
Thank you !

Joffrey