problem with polish characters

[kashiash]

on form when i use few polish characters when i select another control then these characters are replaced by other.
but this happens for specific chars in example when i wrote  jacuś placuś i get jacu[ placu[

in webserver window i see:

GET /updatetodo_tod__opis_value?event=1&value=jacu%C5%9B+placu%C5%9B&_rnd_=0.056571751832962036 HTTP/1.1
Host: 127.0.0.1:88
Connection: keep-alive
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/4.1.249.1064 Safari/532.5
Referer: http://127.0.0.1:88/UpdateToDo
X-Requested-With: XMLHttpRequest
Content-Type: application/x-www-form-urlencoded
Accept: */*
Accept-Encoding: gzip,deflate,sdch
Accept-Language: pl-PL,pl;q=0.8,en-US;q=0.6,en;q=0.4
Accept-Charset: ISO-8859-2,utf-8;q=0.7,*;q=0.3
Cookie: SESSIONID=1860316766


but when i wrote:
jacuś placuś ą (there one more polish char)

than nothing change  - all is ok

GET /updatetodo_tod__opis_value?event=1&value=jacu%C5%9B+placu%C5%9B+%C4%85&_rnd_=0.30056362599134445 HTTP/1.1
Host: 127.0.0.1:88
Connection: keep-alive
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/4.1.249.1064 Safari/532.5
Referer: http://127.0.0.1:88/UpdateToDo
X-Requested-With: XMLHttpRequest
Content-Type: application/x-www-form-urlencoded
Accept: */*
Accept-Encoding: gzip,deflate,sdch
Accept-Language: pl-PL,pl;q=0.8,en-US;q=0.6,en;q=0.4
Accept-Charset: ISO-8859-2,utf-8;q=0.7,*;q=0.3
Cookie: SESSIONID=1860316766

whats wrong ?