mongodb 2.6 mongos start up failed
There was a full set of instructions for the 2.6 upgrade that you don't seem to indicated that you have followed.
In particular are the instructions that indicate you need to process the --upgrade option by launching a single mongos instance while the balancer is switched off:

Start a single 2.6 mongos instance with the configDB pointing to the cluster’s config servers and with the --upgrade option.
To run a mongos with the --upgrade option, you can upgrade an existing mongos instance to 2.6, or if you need to avoid reconfiguring a production mongos instance, you can use a new 2.6 mongos that can reach all the config servers.

mongos --configdb &lt;config servers&gt; --upgrade

As noted there, on completion the following messages should be emitted:
upgrade of config server to v5 successful
Config database is at version v5

So there is an intended upgrade process for the config servers that it would appear you have not followed yet. The full process is in the link at the top of this answer.
Also have a look at the compatibility changes list which has some additional considerations with replica sets and sharding that might possibly apply in your situation.