Hi Kostas,

This is rather unusual, so please:

1. Check ACL (Access) settings of that component.
2. Create a new Super User account (for test period) and try again.
3. Disable Debug mode in Joomla Global Configuration, if you enabled.

Hi Paul, unfortunately none of them helped to fix the problem

Hmm, so far you are the first and only one with that problem so far.

1. Have you tried from both: back-end and front-end editor?
2. Do you have any extra ACL component installed, like ACL Manager or product from Regular Labs etc ?
3. Since when you noticed that, I guess, it worked few weeks ago?
4. Do you have that problem on one website only?
5. Are you able to Save Row (it may help by now)

After spending hours on that problem one of my last attempts was to deactivate the cpanel's ModSecurity and... miraculously it works!!! Μay this lead you to some final conclusion as to why it is happening

ModSecurity - is to protect websites on the server level.

Maybe your hosting support can give more clues. Becuase it's better to have it enabled.