Thanks for your fast response.
Tried all your sugestions and found one that works. It seems that disabling ModSecurity could be a temporary solution althoug not resolving the issue since I loose an important layer of protection for my domain. Before the updates everything worked fine. So it's clear for me that something in the update had messed up.
Is there anything I can do related to this ModSecurity issue, in order to keep a good security and also beeing able to make changes and save pages?
Thank you!