ASM: Failed to convert character

If you know which one is the problematic client and the policy element affected, you are able to allow traffic when it happens using iRule.

Something like this:

when ASM_REQUEST_VIOLATION
{
    set asmdata [ASM::violation_data]
    if {([lindex $asmdata 0] contains "VIOLATION_CHAR_CONV")}
    {
        if { [string tolower [HTTP::header User-Agent]] contains "msie" }
        {
            ASM::unblock
        }
    }
}