Due to changes in the 4.x kernels, there's a problem with the system replying with "icmp unreachable" to incoming IPIP traffic.
This will possible drop incoming traffic, including the RIP broadcasts (resulting in incomplete route tables).
Please switch to an ampr-ripd setup or filter outgoing icmp messages on your WAN interface, using a rule like the one below:
iptables -A OUTPUT -o ethX -p icmp --icmp-type destination-unreachable -m state --state RELATED -j DROP
Optional, you can use version 3.0 and up, providing a netfilter kernel module to solve this problem.