@ in username

[useeliger]

I have found out that a '@' in the username makes problems with Alfresco Share:
If a user has a '@' in his username than he will not be able to see the sites or to search for sites.

[ofrxnz]

there is a similar issue with the user of a dot ( . ) in a username. 

I have also noticed some general weirdness with symbols in passwords too but haven't paid attention

I think the @ symbol may be special in alfresco usernames.  I believe it is for multi-tenancy but don't take my word for it