Daily digest e-mail doen't stop sending

Hi,

today "Subscribe by Email" doesn't stop sending the newsletter. Always with the same content (1 post). Sending time is 6 am. I got an email 8 emails some minutes after 6 am and five after that. At 7.26 am the latest for now.
The log page sais:

23. Januar 2015 06:01 98 Details ? Fertiggestellt
23. Januar 2015 06:01 71 Details ? Fertiggestellt
23. Januar 2015 06:01 71 Details ? Fertiggestellt
23. Januar 2015 06:01 284 Details ? Fertiggestellt
23. Januar 2015 06:01 98 Details ? Fertiggestellt
23. Januar 2015 06:01 123 Details ? Fertiggestellt

Why so many jobs? All are completed but with a different number of receivers. (BTW I only have 75 subscribers in total...).

Until now everything works fine with this plugin. Three days ago I installed WP rocket (caching plugin). Could this be the cause? Or is it a server infrastructure problem?

Thanks for help!

Martin