When you reassign a task from a Coach to a user or team which does not contain the current user you will get an error. You are cutting the branch you are sitting on.
There are two possible resolutions to this situation
1. Send a message to a UCA and let the UCA service reassign with a delay so that the current task does not error out.
2. you can create a filtered team and add the current user to the new team and then reassign.