I've been having this error many times and didn't know why:

PANIC! : Crash adding error, unexpected error found.. [4258]

I thought it was some bug of my Linux distro, but today I realize it starts
showing when more than one shell mirror process is launched, and seems to stop
when all mirrors but one are killed.

Hope this helps!
Great job!

P.S.: I'm using version 3.41 beta 10