Internal Server Error

I often get a web page when ever I try localhost:3000 with the
following message:

Internal Server Error
private method `gsub!' called for #<Class:0xb6d5d764>
WEBrick/1.3.1 (Ruby/1.8.7/2010-01-10) at localhost:3000

I get this thing after I upgraded to Ubuntu 10.04, is some one facing
this same problem. Is there any fix for this?

Thanks in advance.