Global stats at 0%

Hi,

we’ve been using your great plugin for a few years, and have been switching to the version 3 lately.
We have a constant opening/clicked rate of 20%/2.5% over our last newsletters, for a list of 1k subscribers.
The previous newsletter sending (done with version 3.7.4 on the 4th of June) went well, and we had the same kind of stats. However, the last one, done a few days ago (with version 3.7.8 on the 1st of July) shows a constant 0% in both categories.
We’ve been asking a few subscribers, and they did receive and open the newsletter, so we should expect at least some mail openings.
And finally, we are using the MailPoet mail sending service to send our newsletter.

Any idea what could be wrong?