The UnForm clients can return a number of error codes when initializing a job on the server. Here are descriptions of common codes:

997 The server is refusing the client's IP address. The server's ufxxd.ini (version specific) file has an allow= line that list acceptable IP addresses or wildcards, such as 192.168.*.*.
998 The server has all job license slots in use, so a new job can't be started. Often you will see many server log messages as the client automatically retries.
999 The server can't start a process handler within 30 seconds, indicating network configuration problems (often DNS related) or extreme system load.
1024 A client-side timeout communicating with the server. If the server is running slowly, or if the client is waiting to receive a very large job, it may be necessary to increase the client timeout setting with a -timeout n option (n=seconds).
1057 The client could not reach the server at all. This could mean the server isn't running, the client is attempting to connect to the wrong server, the client is trying to connect over the wrong socket/port, or there is a firewall blocking access.

Once a job is started, look for error messages in ##.err files in the UnForm server's temp directory