Hi Thiago,
If you leave the following:
<INPUT TYPE=hidden NAME=SET.category VALUE="$args.category">
<INPUT TYPE=hidden NAME=KEY.category VALUE="$args.category.sym">
The child ticket should populate the Category from the parent ticket.
Is this working?
And when you modified this to set category as "Hardware support" it then gave you this error?
If so can you send the syntax you are testing with?
Thanks,
Scott