Do you guys have any difficulty in opening an account on monetizer. I registered twice and get rejected with the same email below.

Their registration form is very minimal as you can see below and there is no chance to provide an "incorrect" account information when they only ask for name, email, and country. The only final required info field they need is "Where did you hear about us". Is it possible that they expect an essay here while I simple write a sort sentence like "I read about you via multiple interesting threads on STM forum"

I contacted them to ask the reason so that I can provide more but they simply answered that they can't disclose the reason and asked me to register using a new email. So I register with another email and got rejected the same.
For me I just wrote "STM" for where did you hear about us
Sent from my YAL-L21 using STM Forums mobile app
Hard to tell what went wrong there.
Best is to just do what they said and try again with anothert email.
You can also try to send a PM to @Mr Green
Yup, mentioning STM should definitely help.
Let me try to get you some help. Could you please PM me the email you applied to Monetizer with?
And then please drop a note here to let me know to check PM. 
Amy