I think that might be the case.
I've checked, and 7z uses the @ symbol as a parameter to add a list of files to an archive, e.g. @filelist.txt
Maybe it's getting confused and thinking the input file is supposed to be a list...
I have checked the archive to which it is writing, and it is neither solid nor locked.
BTW, I never had this problem under XP. Worked perfectly with the same version of clrmamepro (3.137b), the same DATs and the same ROM input / 7z output files.
It's probably just Windows 7... I've noticed a few strange things.
Thanks for taking the time to look into this though, and thanks for writing such a fantastic tool
If you need any other information, please let me know.