Hi mscheurer,
Thank you for your post and welcome to the Community Forum.
I understand that you received an error message when you were trying to send invoices to your customers. Apologies for the inconvenience have caused you. In this matter, we recommend doing the basic trouble shooting like clearing cache or you can try it in different browser.
If you need further assistance, please feel free to post again. We are happy to assist you.
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Kind Regards,
Cel