Do you have a firewall? Perhaps it is blocking your connection for some reason. Similarly, if you have Little Snitch installed and access to the address is blocked then this is the result you'll get. Even Mac OS X has a built-in firewall, so that is something to consider too.
Something like the above seems quite likely to be the problem, since blocking would usually be done on the address, not the protocol - trying to browse the web site is entirely different and not at all related to the FTP protocol which Yummy FTP uses, nor the SFTP protocol, which is an entirely different protocol again, so I really don't see any way that Yummy FTP could have affected web browsing.
Have you contacted the hosting people to ask them about the strange server message and about this problem? Perhaps they can see something useful at their end.
The only other thing I can suggest to totally eliminate a fault with software (which includes the OS itself as well as any other software you might have installed, including Yummy) is quite drastic - erase the hard drive and re-install the OS

For what it's worth, I understand your frustration, and I am sorry for that, but there are no other reports of such a problem with Yummy. I get all sorts of reports, sometimes about the tiniest little things, so I am certain that if anyone had experienced this issue before I would know about it.