thanks for your help. As I already mentioned in my updated message above, I found the issue and was able to solve it:
My installation is a Bitnami pre-configured AWS-Machine with Mattermost. And there was just a wrong line in the config.json regarding the mysql-connection. It was:
“DataSource”: “username:password@tcp(127.0.0.1:3306)/bitnami_mattermost?charset=utf8”,
correct would be
“DataSource”: “username:password@tcp(127.0.0.1:3306)/bitnami_mattermost?charset=utf8mb4,utf8”,