When compiling the branding section inside an extension, if you specify an image file on a line by its own, if will look for a filename with the line breaks, e.g.
<main_logo _delta="define> my-extension/images/my-logo.png </main_logo>
the file it looks for is \rmy-extension/images/my-logo.png\r which of course doesn't exist and the error message is misleading (it's not easy to see that the filename is wrong since one mentally removes the line breaks).
Line breaks right after the opening tag and right before the closing tag shouldn't be significant.
See also this message in the forum.
Hello Luca,
Thank you for taking the time of creating this ticket. I'll upload it in our bu tracker
Cheers,
Guillaume
Logged as #2883.
Fixed in 3.1.0-beta released today