You have invalid characters in your query string.
On client side you need to escape it before you send request by using
Uri.EscapeUriString(string)
or
Uri.EscapeDataString(string)
Both these methods are limited to 32766 characters in input strings.
The limitation could also be on service side.
If your service is hosted in IIS, I think default max query string is about 4KB.
For IIS 7 you can change maximum allowed url and query string length:
appcmd set config /section:requestfiltering /requestlimits.maxurl:<size in bytes>
and
appcmd set config /section:requestfiltering /requestlimits.maxquerystring:<size in bytes>
You can find more details
here[
^].
You can also use IIS Manager as described
here[
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