The dataserver and repserver each have their own interfaces file. Each server must have the other server in its interfaces file, ie, the repserver must have the dataserver in its interfaces file, the dataserver must have the repserver in its interfaces file.
From the dataserver errorlog ...
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Requested server name not found..
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... it would appear that the repserver is either a) not listed in the dataserver's interfaces file or b) there's a typo with the repserver entry in the dataserver's interfaces file.
In your dataserver interfaces file, make sure you have a valid entry for the repserver; then try to start the repagent again and see if it's able to connect to the repserver.
NOTE: You can verify the accuracy of the dataserver's interfaces file contents by sourcing the dataserver's $SYBASE/SYBASE.sh file and then try to use isql to connect to the repserver.