After the "latest" update one of our clients asked us to figure out the issue and it turns out it is related to the mail settings.
Basically all e-mails were being sent with blank text/no body.
Go to "Setup -> General Settings -> Mail" and just click on "Save" and it will work again.
Note that this assumes you didn't break things yourself by changing any settings. WHMCS may tell you to use "8bit encoding" but you probably already have that as the default. It seems that something in the database gets fixed when saving the settings again even if they're already correct/unchanged.