Specific website(s) not working via specific network

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Hi Community,

First time poster for almost anything, I'm hoping someone can help to point me to the root cause of some odd issues I've been trying to get to the bottom of please.

At least two websites, are refusing to work via a corporate MPLS with dual internet breakouts, traffic on the firewalls from either breakout seems to succeed just fine yet the page will not load. On any number of other internet breakouts, non-corporate, from same client(s), loads perfectly fine without issue, it is two websites from same server/hosted supplier it seems that is effected.

I can try multiple DNS, internal and external, to no affect from within corporate LAN, name resolution works fine in all cases.

I've sought support from web page owner to seek if they are blocking and I'm told they are not.

I've sought support from maintainer of MPLS and firewalls and am told as it is not a firewall issue that they cannot help.

I've shared captures of both working and non-working cases with these parties but had little response, I'm no Wireshark expert myself and cannot see the root of the problem, I feel it must be a block at the website providers end but can't prove this, can anyone help shed some light on it please for me?

Not sure what this 'points' system is so if anyone can advise on how to share captures that would be appreciated also.

Many, many thanks in advance for any help you are able to provide.