I configured a UserParameter called rofs (stands for read-only-file-systems, but that isn't important). Then I configured it as an item, a process that has worked seamlessly many times in the past. This time, I witnessed a situation I hadn't encountered before: The server kept on showing '-' for the value, no values were received, even after 30 minutes (refresh was 300 seconds). The item also didn't switch to not supported.

The steps I took to debug the situation:

• I debugged solely on the v1.6.1 agent
• I checked the agent using telnet and the parameter name
• I replaced the command for the UserParameter with "echo 99"
• I created additional copies of the UserParameter called rofsa, rofsab, and rofsabc

And the result is that the agent sends the value back properly: ZBXD0 (0 was the proper return value for the original command) and that the additional items reacted as follows:
rofs and rofsa only ever displayed '-' and never contained any values in their histories
rofsab and rofsabc responded normally and displayed values as expected.

This leads me to the conclusion that the server (and not the agent) has a problem with UserParameters that have a length of four characters or less.