The guess is that you are using a different version of the tool than previously used (?). You could try the prior version that worked, and we can try to fix the broken version meanwhile.
Few questions to isolate the problem:
Which Galaxy server are you working at? URL or describe if other
What is the full tool name and version (find this at the very top of the tool form).
Is this tool on Galaxy? I’m having trouble finding it, but am also interested in using it. (By the way, I’m using the European server but could switch to the main one if need be.)