Hi mscheurer,
Thank you again for getting back to us. I understand how important this is to you.
I check back on your concern and look for the probable reason the issue persists. This issue can happen if the link for invoices has a problem. You can try to delete the invoice and re-send it to your client. This can fix the issue.
Feel free to post again if you need help or in the future. We are happy to assist you.
If my response has answered your enquiry, please click "Accept as Solution" to assist other users find this information.
Kind Regards,
Cel