Thanks so much for your time dedicated to support me in fixing this difficulty with my course.
Well..let me explain you the facts before I received the message attached.
1. I could enter to the course content and then I went to the configure option to change dates of the course as I did in the past without any problem
2. I tried to press the key at the end of the content page in order to fix/save the new dates configuration of the course content, however it appeared the message I am showing you.
3. Then I start to review which possible mistake I have made and I realized that I was not configured as administrator of the course. Then I proceed to do it. I thought that this could be the problem.
4. I did a test again, doing the same as numbers 1 and 2, but the same message appeared again.
5. Then I thought I have to clean all the files and message from previours participants of the course, erasing all them.
6.. I test again and I tried to re-configure the course but again the same message apperared.
7. I have tested other courses to see if I have the same problem and no, I have this problem only in this course.
I hope that this comment will clarify you about this issue.