What does the message PRN: FEC::ReceiverError mean?

FEC stands for Fast Ethernet Controller (100mb LAN). The ReceiverError line is followed by a number that denotes the exact problem.

Basically it is stating that the system received a packet that it considers wrong or corrupt in some way or perhaps there was a collision so it threw it away, the packet would then have been re-sent. This is does not normally indicate a problem and is nothing to worry about unless the error's are streaming in the trace. See Decoding FEC errors.

What does the message PRN: UDP: Sending from indeterminate address to 0a000003 3851 mean?

The port number 3851 at the end indicates that the system is looking for an IP Office Voicemail Server.

If your system is not using voicemail, remove the entry in the Voicemail IP Address field, found on the Voicemail tab of the System form in the system configuration.