Strange behaviour logo (.PNG format)

Hi there. I searched the knowledge base but coudn’t find an answer to my problem

I created a logo for my client in Illustrator. I saved it as PNG format to preserve transparency. But When I upload the logo in your theme’s basic options / upload logo. The logo then shows very strange. It looks like the upload system is broken up the logo. I tried it on a different domain with a different theme and within there the logo shows up just fine. I fixed it to add a white background to to the image and switched off the transparency of the header