Fatal error: Interface 'Mailchimp\MailchimpApiInterface' not found in

I'm running into this as well, and it looks like there are actually a few issues. Namely, the Drupal 7 version of the Mailchimp module is no longer in sync with the recent API changes in the Mailchimp 3.x library.

This issue in particular can be fixed by adding 'src/MailchimpApiInterface.php', to the hook_libraries_info() implementation. That will clear up the error in question.

Unfortunately there are quite a few more changes that will be need to made in order to bring this module in line with the Drupal 8+ versions. I'm hopeful that the maintainers will recognize that the Drupal 7 version is still in use on live sites, and with ours we really need the ability to tag Audience members / accounts for promotional marketing emails, so having the module updated would be a huge help. I'll try to create a new meta Issue with some of the tasks that I'm finding as I dig in to make the module compatible with the latest MC API Library.

@TorgosPizza, I just wanted to let you know we're working on this. I'm looking at a temporary fix where we just downgrade the required version of the Mailchimp API library, to get things working again while we do the heavier lifting of getting the D7 version working with that library's 3.x version. More on this today or tomorrow.

Okay, for the 7.x-5.x branch of this project, locking the required version of thinkshout/mailchimp-api-php at 2.1.3 (but allowing any version between 1.0.8 and 2.1.3, inclusive, so that people who are happy with version aren't forced to upgrade).