The online reservation for Shinjuku to Fujisan 5th Station and return bus does not seem to work. ...

Hi.

I am sorry I gave you wrong information.
The bus for Mt. Fuji 5th station will be available from July 1. The timetable for 2016 is not out, yet. It should be available on June 1. In your case, you should be able to reserve a seat from June 14 as you can only make a reservation from one month prior to the departure date from the following site.
http://highway-buses.jp/

Thank you!

Hi.

First of all, the online reservation system has been working now.
http://highway-buses.jp/

You can make a reservation from one month ahead of the departure date. However, as mentioned in the previous comment by Saffy, the bus for Mt. Fuji 5th station is running on weekends and holidays only.

I have climbed Mt. Fuji and watched the sunrise 4 years ago as well. I still remember how beautiful it was.
I hope you can make it again.

Good luck!

Hi there!

Oh no, again!?

A system error occurred during the Holiday in Japan, but I heard it's all fixed.
So I double checked to make sure on the phone, it seems to be working properly, they say.

But please let me try again tomorrow morning to make it sure, I'll make one more call during the office hours.

AND, I think you've already know this, please make sure that this bus runs only on weekends (sat and sun).

Hope this can be your help.