This thread actually started in 2018. Constant Contact engineering still doesn't have this fixed? I uploaded a gif successfully, changed the color and went to upload the new version and bam got the error. This work around did work for me> Convert the GIF to an MP4 video using https://ezgif.com/gif-to-mp4 , then convert the MP4 back to a GIF (Leave the default settings throughout the process). But I agree with everyone on this thread. Constant Contact needs to fix the issue.
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