ISSUE SUMMARY:
User tried to post this video
www.youtube.com/watch?v=1kVtz9cjt2k
When he paste this URL, thumb is there but when he tried to post it,nothing is happening.

STEPS TO REPLICATE:
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RESULT
EXPECTED RESULT
BROWSER

Hi Drazen,

what is your administrator URL, seem you using URL security at administrator URL.
please edit your first post at this topic. and update any credentials information at site info form.

the issue can be related with:
1. maybe do you have any redirect setup at htaccess or else? try to disable it.
2. any security for some request?

Regards

I updated admin URL. Everything is default, there is no special security requests

Hi Drazen,

I got forbidden error (403) something blocked the request from that URL. Because this issue only happen on your site with some video only, It must be related with server security things.

please asking your server support and have a look any error logs for 403.

Regards