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 )

Flutter

Application ID

EF7D3338-C2D8-3CB9-FF63-6455EEC5B800

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 [mark@backendless.com]. Email template ‘DRK Negativ’ : Invalid Addresses
2021-04-02 00:59:47,904 | Global logger | ERROR | Send custom template ‘DRK Negativ’ for ‘mark@backendless.com’ 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:
mark@backendless.com

MainFile:
main.txt (4.7 KB)