Hi Ranjeet,
The problem here might be of the internet domain name. I came to conclusion because of following reasons:
1. while creating subtype, we have to provide internal name.Internal name is unique for the subtype. The reverse of internet domain name for the owning organization is automatically pre-appended to the name we enter there.
2. internal name can only contain letters, numbers, periods and underscores
3. so if your internet domain names contains any other charecters than mentioned in point 2 then it creates this issue/ error which you mentioned.
I have also tried to create the document subtype but I got the same error.
I am not sure what is the resolution of this issue, if I get I will post it here.