Hi @razvan_iacob,
A bit hard for me to figure the root cause without access to the ENV. However, if I remember correctly, you ENV is of 11.n.n and you've only upgraded the HTML5 player. While this can often work out just fine, there are often changes in the player or Studio that require related changes in the server [the other way around can sometimes also be true]. Therefore, my suggestion would be to upgrade the entire system to the latest version [12.9.0]. This is more than likely to solve all your issues.