nonce invalid

We’ve been having an issue with Advanced Ads for some time now:

When I try to change the weight of individual ads within an ad group, or when I try to add a new ad to the group, Advanced Ads does not save the changes and I get a message “nonce invalid”.

As soon as I click “save” on the weights table, WordPress displays message (“invalid nonce”) in the top right corner, but the change is not saved.
(Note: At first, WordPress acts as if the change was saved. But when I refresh the page with the group and check the weights again, the changes are gone.)

The issue already existed before the latest update, but I left it for the time being because I didn’t urgently need the feature and was hoping the update would fix it. However, I just checked again and the problem still persists. We run a pretty up to date wordpress version and the latest version of “Advanced Ads” (2.0.9). We oppened a ticket as well, but no response (though we use the paid pro licence).

We also had a developer take a look who checked and reset file permissions (did not solve the issue). He assumes the error is in the plugin.

I currently need to make a few changes to the ad groups.: This is urgent! Please take a look!

Best regards,
Sebastian

The page I need help with: [log in to see the link]