TYPO3 Core

I have installed typo3 6.2.6 and the introduction package with Bootstrap. Everything was in order, but suddenly it is not possible to "Add image" to a content element from fileadmin, because this generates an error message "Error 404 - not found". Even if the image really exist in fileadmin. I could upload pictures to fileadmin, but I cannot "Add image" to a content element. If I try to "Edit content" in a CSS file in fileadmin, the file comes up fine, but when I try to save it I get the message "Page not found - because tce_file.php not found". It seems OK to upload files to fileadmin, but impossible to use them from a content element or edit files. I upgraded the website to typo3 6.2.7, but the same problem continues. Is it something I could check in order to find out the reason for this behaviour? The only extensions I have imported is bootstrap_package, t3colorbox and realurl.
Regards,
Gunnar Jonsson