I tried with an image of 740 x 89 and made the image’s name just 6 letters long and it still returns the same error.
Whatever image I set as the header image causes that error, no matter how small.
Further-info:
style.css stopped working when I added the following code:
.select2-container .select2-arrow ::after{color:#fff}
editor-style.css stopped working when I added:
.page-id-336::-webkit-input-placeholder {color: white;}
.page-id-336:-moz-placeholder { /* Firefox 18- */ color: white;}
.page-id-336::-moz-placeholder { /* Firefox 19+ */ color: white;}
.page-id-336:-ms-input-placeholder { color: white;}
