You still haven't indicated whether or not CSS buttons are in use for the store. OPC's /tpl_modules_opc_submit_block.php (distributed in /template_default and possibly overridden by YOUR_TEMPLATE) uses the built-in zen_image_button function to create that button.
You might try comparing the use in that OPC template-file against others provided by your template.
Back in April 2020 it was established that customers checking out as a guest, without already having an account, would receive a text-only checkout email (https://github.com/lat9/one_page_checkout/issues/244).
Has anything changed in the interim or does that restriction still apply?
Simon
Just noticed a strange thing, I was looking on the website checking listings for an order, and got an error 500 on every page. Or refresh. In private session or another browser I could access the site. But not in the affected browser session. Only other thing is I Was logged into admin at the same time. I could only see several logs like this:-
I turned off one page checkout for now in case its that, though got a feeling it might be due to ip address logged into admin and somehow its blocking in same session on existing browser. I didn't see anyone else with same issue, I saw a code edit I applied to make this log disappear though. In case that helps.PHP Fatal error: Uncaught Error: Call to a member function guestCheckoutEnabled() on int in /includes/classes/observers/class.checkout_one_observer.php:216