My server crashed and webx.log shows "webx-go terminated by signal 11".

A signal 11 is a memory fault. While it is certainly possible that this is caused by a problem with Web Crossing, more often than not this is indicative of a hardware issue or a problem with the kernel of the OS.

Here's an interesting site that covers Signal 11 as it pertains to compiling Linux, but the concepts are equally applicable here:

http://www.bitwizard.nl/sig11/