Mootools error

Moo tool errors will not let me save in the back end

error seems to be on line 310 of the mootools core file

Is it in default installation (quickstart), or You have made some changes to it (uploaded 3rd party plugins etc)?

quick start
two extensions loaded at this point Akeeda backup and JCE

got me thinking.. uninstalled akeeda backup and no problems so far

Sometimes 3rd party plugins mess with the code, installs their own version of basic libraries etc - in most cases trying to turne them off and checking allows to find source of a problem.