Invalid Token page displays when trying to login - Joomla! Forum - community, help and support

hello, i'm using joomla 2.5.7 since 1 year on 2 noprofit sites.
one of them flyingsclub site http://www.gvmp.aero, other 1 school's site http://www.didacti.ch (i'm flight instructor , teacher electronic engineer background)

the sites run reliably , real problem that, when user tries login (for downloading private documents), he/she gets redirected white "invalid token page". happens school's site http://www.didacti.ch.

on site, use core login module, joomla cache disabled (as suggested post). use jat3 template.

the user think website crash, , difficult (and not professionnal) explain them clear browser's cache (wich sometime solves issue)

any idea how avoid problem?

thanks lot

easytherm wrote:... , difficult (and not professionnal) explain them clear browser's cache (wich sometime solves issue)

any idea how avoid problem?
...

if problem exists on clients computer , not yours problem. si in fact unprofessional let them think otherwise , not explain how clear browser cache.