FlexCard Preview not working

I am not able to preview any FlexCard in my Org. The preview button gets stuck on the below screen. 

I have tried adding Remote Site Settings as per the article below, but it hasn't helped.  

https://help.salesforce.com/s/articleView?id=xcloud.os_manually_update_remote_site_settings_for_flexcards_43272.htm&type=5

Any else who has faced similar issue? Would appreciate any help. Thanks