annoying little white border line appeared in Gif animation

Hello!

With photoshop CC 2018 I created a gif animation, clicking on "create video timeline" and then using the clock icon with the "transform", changing the w value to 1% at frame 100 . The original image was this png file: https://postimg.org/image/iq7f73hwb/  , the result this gif: https://media.giphy.com/media/gXPl17sWWZ0HndMPQW/giphy.gif

Both have and should have transparent background, I am 99% happy with the result. Unfortunately this annoying detail, at some point a thin white outside boarder around the question mark appeared. To demonstrate (since it might not be visible with the two images above) I saved a single frame, put it in front of a green background (for some unknown reason it came out dark grey instead of green, but that is not the point here). https://postimg.org/image/f99yhid97/

The thin white line is annoying since I will use the animation with another background later. I hope it is just a little check/uncheck a boarder setting or using an svg or whatever filetype as the origininal image or something with safe settings. Any ideas? Thanks in advance!