Hello @IoannaAlexopoul,
When you go to the Moderate Quiz page, if there any attempts listed and you are receiving this message, you may need to escalate this issue to your institution's Canvas support team. This is an issue that has been reported before and I do not see an answer to the issue.
Have you tried accessing the quiz log via the SpeedGrader? I am not sure if that would show anything different, but it is worth a try.
Also, may I ask what you are using the log for? I ask because the data you seek might also be available via the Quiz Statistics.
Regards,
James
