I think the answer is fairly obvious from my reading of the first two posts.
The OP is almost certainly correct that the site author has assumed (wrongly) that Opera is mobile only, and has set their site to serve the mobile version if Opera is detected from its user-agent string.
The OP has used the User-Agent Switcher extension to mask Opera as being another browser, and the site now works correctly.
The site owner just needs to be told of their mistake, that's the cause!
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