Send error on android devices

Some users of my site reported to me, that they can’t submit my forms from their android devices.

Because I got no android device I tested this behavior with Google Chrome Developer Tools selection a Nexus phone and it’s true that my forms responsing with “There was an error trying to send your message….” after submitting. Testing this with other devices/browser the form is submitted correctly.

There are no javascript nor any debug errors.

After hours of testing the eMail address settings, SMTP Mailing, switch back to an older CF7 version and reinstalling CF7 v4.8.1, changing to a WP standard theme and deactivating all other plugins – all with no success – I figured out that there is an event within CF7 that gave me a response after submitting:

document.addEventListener( 'wpcf7submit', function( event ) {
    console.log(event)
}, false );

In the detail node is see status:"spam" if submitting from an android device. This value is mail_sent if submitting from an other device/browser with the same content on each form field.

Did anybody having an idea, why it’s marked as spam, if it’s an android device? Is there any solution to solve this problem?

Thanks!!!