The meaning of the message is that one side of the IPSEC tunnel received a packet with an invalid SPI. The SPI (Security Parameter Index) is used to identify the SA (Security Association) of the packet - which contains the information needed to handl...

Hey, this is probably a small hardware issue, which is well explained in this thread. If this message appears only once (or very seldom) it is probably insignificant. However, if it starts to repeat more often, especially with different "locations", ...