What are the allowable characters in a name?

Commas are not allowed as they are used to delimit lists of usernames. There are no other restricted characters to my knowledge. But of course you can impose your own requirements on usernames by defining a regex as Brogan pointed out.
 
This function verifies usernames:

XenForo_DataWriter_User::_verifyUsername

There are several restrictions, many of which are user-defined such as censored words.

What error message are you getting? That will help to know which check is failing.
 
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