Attachments get uploaded to server, but are missing in emails

Dear forum members,
I have a strange problem with “contact form 7” (5.1.9) and “Drag and Drop Multiple File Upload – Contact Form 7”. Everything worked fine for months, but a few days ago the form stopped to transfer attached files to the sent emails.

In other words: when the form is filled out and submitted, it uploads the attached file(s) to the temp folder, the success message is shown, the mail is sent and I can see the uploaded files in the /wp-content/uploads/wp_dndcf7_uploads/wpcf7-files/ folder. So the form itself seems to be working. But the received eMails only contain the text of the form fields, but not the attached files!

I checked everything, even the short code for the attachments in the email-tab of cf7 is correct (otherwise it wouln’t even upload the files to the temp-folder).

When I check the submitted forms via the “Flamingo”-plugin, I can see the mails text AND the correct link to the temp-folder, from where I even can open the uploaded files.

But when I call up my mails, the do not contain the attached file. Even the confirmation copy to the sender, which is in a complete different mailing-domain, doe not contain the files, so I can exclude an antivirus software or some network or firewall rule from being the reason.

I tried to downgrade cf7 temporarily to a former version, but this didn’t change anything. So my guess is, the problem does not arise from the plugin itself. But what could it be then?

Besides a related question: the attachments in the cf7-temp-upload folder seem to get deleted from time to time automatically. So after a while I even can’t get access to the attachments in that way any more. Does anyone know where to control this behaviour?

Any help greatly appreciated!
THX Christian