Invalid Security Token when saving from admin panel

Hi all,

I'm getting 'Invalid Security Token' when trying to save a page.

situation:
Custom template tweaked from antimatter: features.
all seems to be working fine, I can gui edit the values and save for a page with less than 150 entries,
but when trying to save the same page with over 180 entries... I get : 'Invalid Security Token' on admin pages.

any pointers?

more info:
Grav v1.4.2 - Admin v1.7.4
front matter structure:

title: 
sections:
    -
        id: 
        name: 
        title: 
        rubi: 
        email: 
        ext: 
    -

dedicated yaml is set and working fine.
data entities are single quoted ie. id: '012345'
and utf-8 Japanese text is used.

able to save filesize: 4.0 K
unable to save filesize: 28 K