There was a problem with Growthexperiments-help-panel-question-subject-template-with-title ("Help panel question on $1"). If $1 is "Category:something" it will be sometihing like  ("หมวดหมู่" is alia for Category) and add page to category. I would suggest to add ":" to escape category magic like [[$1]] → [[:$1]]
[[$1]] → [[:$1]]
Thank you for the report. I have submitted patch Gerrit:693604, waiting for review now.
Above patch was approved and will go live this or next week.
Thank a lots :-)