My educated guess:
in reply to Problem with qx/system and long urls
Windows CMD.EXE has a limit on the total # of bytes for environment variables and the names of the variables: 32,767. Your URL has 16K bytes, and it's put in the environment more than once (request_uri, path, query_string). So the process can't be created with that big an environment.
Solution: use HTTP POST method instead of GET.
Although the IETF standards don't specify a maximum length for URLs, RFC2616 does say "Note: Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations might not properly support these lengths."
FYI: Microsoft IE and IIS limit URLs to about 2K.