Thank you for the reply.
The version is Moodle 2.8.5+ and here is the error message:
Debug info: [dataroot]/filedir/6d/74/6d741064f0579af5c89bfdf7730ca0d02d1d1030
Error code: storedfilecannotread
- line 187 of /lib/filestorage/stored_file.php: file_exception thrown
- line 881 of /lib/filestorage/stored_file.php: call to stored_file->update()
- line 424 of /lib/filelib.php: call to stored_file->set_source()
- line 193 of /lib/filelib.php: call to file_prepare_draft_area()
- line 91 of /course/editsection_form.php: call to file_prepare_standard_editor()
- line 56 of /course/editsection.php: call to editsection_form->set_data()
I checked moodledata folder and it seems that file actually don't exist. Can't really understand why.