According to the "Troubleshooting Switch Port and Interface Problem"-Document from Cisco.com, the error is described as follows:
"A Symbol error means the interface detects an undefined (invalid) Symbol received. Small amounts of symbol errors can be ignored. Large amounts of symbol errors can indicate a bad device, cable, or hardware."
You can find the document here:
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008015bfd6.shtml
Please rate if the post helps!
Regards,
Michael