Order mails is not being sent out

For some reason no order mails are being sent out. I have checked the email settings and all other mails from my Contao website works, "activate your account", "new password" and "newsletter". I also entered a valid email address to the Store configuration (not an alias) and I use the same address in the module (tried different emails too but it made no difference). No mails are being sent to the store admin or to the customer.

Someone who has an idea?

Do you have your email templates set up correctly in the store configuration? As well as a valid email address loading in your checkout module?

What version of Isotope? I've had this occasionally (not on all occasions), Andreas fixed that problem very recently. Perhaps it is related?