I was completing a 40-term quiz and reached the final question, which I answered incorrectly and needed to type out. I typed out the answer and hit confirm, and the quiz timed out and sent me back to the menu. I'm not sure what triggered this issue, but I'm using Firefox on Windows 11 and was switching between numerous tabs. The quiz was open for a while, so it might've timed out in the background.
That's probably the case, then. I would go into renshuu's settings and change the "new day" for the schedules to something in the middle of the night (for you), so there is no chance of it overlapping.