Hello Mario,
In general DTP or VTP frames from switches can be a reason in LAN interfaces.
For the WAN interface that is a LAN too:
in addition
if the router in the other side is running IS-IS and you router doesn't the IS_IS hello pdus count as unknown in the order of 1 every 10 seconds.
Is the rate of increments constant over time or do you see spikes of unknown protocol count number ?
This could help also.
It looks like you see some peaks over time not a regular increment.
However, if the router declares the packets on the LAN side dropped for unknown protocol there shouldn't be a direct correlation with the packets that increment the counters on the WAN interface.
There can be hosts/routers on both sides that are running IPv6 and your router is single stack IPv4 only ando so IPv6 packets can count as unknown protocol dropped ?
Hope to help
Giuseppe