The logfile provided only shows recording attempts at 60fps, with under 1% encoding lag. As you are using x264, this means that your CPU is over-loaded. As you're using the Ultrafast preset, it's likely whatever game you're trying to record is fully-loading your system.
We'll need a logfile from a 120fps recording session to give insight on what's going wrong, however, be aware that recording at above 60fps is not a supported use-case and is not guaranteed to work.
You might try switching over to the AMF encoder, but will need to update your GPU drivers first as they are extremely outdated to the point the current drivers on your system do not support AMF calls.