"Debugging and troubleshooting", "List of MyChat Client errors", "Error #0293".
 
Error

Error #0293: impossible to rename the personal user group, such name already exists.
 

Error description

You can't rename your personal group, because you already have the group with the same name.

 

Notice, that group names can be written in lower/upper case, but the names "My Group" and "my group" are considered the same. Try to stick to this rule.
 
Problem solution

Choose another name for a group, or change the name of the old one first to avoid dublicating.