Dear @vege44
Thank you for the clarification. I just tried the GUI interface to disable the Thumbnails generation and it worked very smoothly. PFA the screenshot.
Having said that, I am afraid your NAS may have some issue and we would like to review the same and help to fix. request you to kindly raise a Support ticket so that our tech support team can review the logs and try to fix the problem.
You may submit the ticket using the following link: QNAP Account Center
Thank you.