Had this similar problem with Whatsapp...
Solution :
1. Go to android market.
2. Option/Menu key>my apps
3. Scroll and select WhatsApp Mesenger > hit update
Once updated, try to use it once again.
Have a try. It works for me.
BTW, I had internal memory full issue with my Arc..
It prompted internal memory full earlier this morning..
Cleared some cache file and moved a few more apps to SD..
Managed to dig 45Mb free on my internal right now..
Is it normal to have 40-30Mb free on internal..?
Tanak la root2 ni.. risau..

1) any way to remove the slider lockscreen as i am using the pattern lock too? Need to do 2 job la