Our latest server version is v5.20: https://docs.mattermost.com/administration/changelog.html#release-v5-20-feature-release.
Did you read all the important upgrade notes when updating to v5.2: https://docs.mattermost.com/administration/important-upgrade-notes.html?
Would you be able to check if the suggestions on this post might help Gfycat gifs not playing automatically after upgrade from 5.3 to 5.19?