Sending email with template doesn't work / without template is working

Hey, I try to send an email with an included template. As I already discussed with @markpiller something in my code has to be wrong. When I try to send an email without a template, everything works fine. Thanks for any kind of help.

Backendless Version (3.x / 5.x, Online / Managed / Pro )

backendless_sdk: ^6.3.0 (free version)

Client SDK (REST / Android / Objective-C / Swift / JS )


Application ID


Expected Behavior

Sending an email with the included template.

Actual Behavior

Following error in Backendless logs: (no error in vs code)
2021-04-02 00:59:47,898 | SMTP Logger | ERROR | Error occurred during template-based email sending. Subject ‘Das Ergebnis Ihres Corona-Schnelltests ist da!’. Recipients []. Email template ‘DRK Negativ’ : Invalid Addresses
2021-04-02 00:59:47,904 | Global logger | ERROR | Send custom template ‘DRK Negativ’ for ‘’ was failed : Invalid Addresses
2021-04-02 00:59:47,910 | Global logger | ERROR | Result of sending messages using the template ‘DRK Negativ’ totalCount/failedCount:1/1 : Failed emails:

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