After upgrading the cluster to 2.4.1 we started having lots of errors like:
pymongo.errors.ConnectionFailure: could not connect to anime1.lan:27017: (99, 'Cannot assign requested address'), could not connect to anime1.lan:27017: (99, 'Cannot assign requested address')

I have checked and the master is UP.
ulimit -n 64000
(like in docs)

cat /proc/21909/limits | grep "open file"
Max open files 64000 64000 files