I'm getting the error -
ORA-03113 end-of-file on communication channel
When I attempt to create a new DBLink or test an existing one.

I've searched my server for the alert.log, but it doesn't exist, I read somewhere to check the User Dump but don't know where to find it, or what it is called exactly.
Does anyone have any ideas on what the problem could be, what to check and where etc.?

Regards Al