Need some help...
I do have several websites using cmsms 1.11.4. I have been trying to upgrade to newer version using several files (full install of only differential installation 1.11.9). The installation worked well...instalaltion successful when going through the different steps of upgrading...Once finished, the webpages are working well..the only problem is when I am trying to connect to the administrator site, I got a blank page with error server 500

When adding to the config.php the line:
$config['debug'] = true;
then I can again access to the administrator and I don't see any problem on the log lines below. I update all modules and performed all maintenance operations..but not change I cannot access to administrator when debug= false
Don't know if there is a link but in administrator the image manager causes also an error 500 in the "directory explorer" but I can still add some new images
Thanks in advance for your help