Probably a good time to change the belt if it hasn't been changed. You won't notice the cost of the belt so much as compared to the cost of the alternator...
Very good chance it caused the error codes. Failing batteries are also known to start giving random fault messages flagging the transmission etc.
There's a bit of info how to replace the alternator here:
http://www.aulro.com/afvb/d3-d4-rrs/...placement.html
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