What could be the cause for these strange UFW block entries in my syslog?

I believe you have some service on your local network that is advertising itself or looking for clients. It's your "default deny incoming" rule that's blocking this traffic. Yours looks much like persistent noise I see on my home network, caused by a Multicast DNS server in my router.

mDNS multicasts to 224.0.0.251, so that's not what yours is. You have something multicasting to 224.0.0.1, a generic "all hosts" address. Couldn't tell you what it is from that, but from the subnet address (x.x.x.1) I'm guessing your router is the source.

My guess is that it's the local router asking if there are any hosts interested in receiving multicasts - nothing to worry about (see [1])

You can prevent the creation of new log entries with this command (works for me on 13.10 64 bit):

sudo ufw deny from 192.168.178.1 to 224.0.0.1

I cannot say if this will have negative side effects for your networked device(s).

If you want to see UFW firewall rules, type

sudo ufw status numbered

and if you want to get rid of it write

sudo ufw delete 3

if the rule you want to delete is rule number 3. Work your upwards in the rule list.