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Your LimeSurvey version: LimeSurvey Community Edition Version 6.4.0+231218 
Own server or LimeSurvey hosting: On Premise
Survey theme/template: Vanilla
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Hello, 
I installed pdfreport & limeMpdf plugins on Limesurvey and it worked very well - perfect for my needs. 
I tried to add a logo in the header.twig  and following the documentation add  this    in the twig file (and uploaded the image  in the folder "assets".
Unfortunatly I still have a small red cross instead of the logo. I guess it's something with the path (or with syntax..) , I tried full URL, external img with full URL... nothing worked.)

If someone has an idea of whhat I am doing wrong, it will be really great. 

Thanks in advance for the help ! 

For external full url : maybe your server disable this.

You can use complete directory name, for example
/var/www/limesurvey/upload/theme/files/logo_pdf.png

With imageSrc

/var/www/limesurvey{{ imageSrc('./plugins/limeMpdf/assets/logo_pdf.png')}}

Adding you base directory