Step to reproduce:
Please take a look into your error_log for more information why you are getting 403s. This is not a Matomo problem.
This is definitely some server configuration issue. Maybe some security mod is active or something? HTTP 403 means forbidden. So something seems to block it. As @fdellwing mentioned I recommend you have a look at the error log. If you can't find the solution, maybe see if someone can help in the forum: https://forum.matomo.org
I try some suggestions in here:
After I change it to my own URL it works and can serve the html.
The problem are this not always happens, but mostly happens.
So, with enough patience to refresh the page I can get it working normally again.
As you mentions to check error_log, there is no specific error, only have shown the URL requests got 403.
I found some cause which sometimes Matomo will fetch proxy IP instead of original IP that can cause server block the proxy IP.
Could you help to take a look about it @tsteur
Currently if I whitelist the proxy IP it will works but I sometimes it can have multi or different proxy IP which can be a lot.
I also already follow this guide:
Thank you for your help.
I can't really add more besides the guide unfortunately. Those setups can be quite complicated and different from each other etc