After I upgraded to 3.9.1, I suddenly got an error that MailChimp Immediate Send (MCIS) couldn't find DeMomentSomTres Tools.
I got this message when I tried to open MCIS' options page.
I installed DeMomentSomTres Tools separately. After I did that, I'm now able to open the options page, but it looks a bit weird and no e-mails are sent when I create new posts.
If I try to insert an invalid MailChimp API key, I don't get an error message, so it doesn't seem to actually connect to MailChimp anymore.
I have tried uninstalling and reinstalling the MCIS plugin, but it doesn't make any difference.