unable to add comments

Hi All

I am not much aware of planner. We use exchange 2016 hybrid environment. we create users in onprem and migrate to online. we use E5 license.
One of my user is facing issue with planner. User is a member of a group that has access to planner lets say test1.The user is facing issue adding comments. when user clicks save, screen pops up and comments disappear. i suspect user can create a task and add comment but not to all tasks. user is also getting error 415, unsupported media type in their team Planner