'png_compression_level should be an integer from 0 to 9' upon uploading image First time we see this: client tried to upload a specific PNG file (that a first sight looks ok) and Craft CMS returned the error 'png_compression_level should be an integer from 0 to 9'
This turned out to be a bug and the fix will be in the next release (current version is 3.5.15.1).