I don't know about iRule error message.

From the format man page:

i

Convert integer to signed decimal string; the integer may either be in decimal, in octal (with a leading 0) or in hexadecimal (with a leading 0x).Looks like the leading 0 indicates an octal value, and 8 & 9 are not legal octal digits.

I couldn't figure out a way to get format to solve this problem, but here's something that should work around the issue until a better solution is found:

when CLIENT_DATA {
  if {[UDP::payload 2] < 10 }{
    set index1 [substr [UDP::payload 2] 1 1]
  } else {
    set index1 [UDP::payload 2]
  }
  set div1 [expr {$index1%2}] 
  log local0. "index_6001: $index1"
  log local0. "div_6001: $div1"
  if {$div1 == 0} {
    pool pool_s1_6001
  } else {
    pool pool_s2_6001
  }
}

HTH, and please post back (anybody!) if you find a cleaner solution.

/deb