I was having issues before trying to run an scheduled task from the UI pressing the "Run now" button because nothing was happening and I was getting no feedback at all.

After that I realized that I was entering the path to php setting with an empty space at the beginning " /usr/bin/php". The form to add the setting seems to validate if the path is correct but it let's you save the value anyway.

Then I looked at the code and I saw that in this line it compares $CFG->pathtophp using the trim() method but then it returns the value without using trim(). 

Because Moodle allows to store the value like this, then the "Run now" button never works.