SIP error message

Hi, I have installed FreePBX 12 and Asterisk 13. We are seeing the following error

[2014-11-12 11:05:17] ERROR[13387] chan_sip.c: Serious Network Trouble; __sip_xmit returns error for pkt data
[2014-11-12 11:05:31] ERROR[13387] chan_sip.c: Serious Network Trouble; __sip_xmit returns error for pkt data
[2014-11-12 11:05:45] ERROR[13387] chan_sip.c: Serious Network Trouble; __sip_xmit returns error for pkt data
[2014-11-12 11:05:59] ERROR[13387] chan_sip.c: Serious Network Trouble; __sip_xmit returns error for pkt data

The interesting part is that the error messages are coming at exact 14 second intervals. It looks like this is not a “real” network issue (I would expect that to be more random). Any ideas of what is going on?

This is my first post to the forum so thanks in advance for help.