New shop intermittently throwing up " Whoops your session has expired" on all actions

The first domain is historic and was supplied and hosted by a domain name provider who do not offer SQL, PHP etc. The second domain was for a separate project that req'd SQL, so it was purchased from a different hosting company that included those services. The two were combined some years ago by simple forwarding from one to the other, and has worked fine until now. Our present situation is that the first domain name is preferred and dominent, but the second domain has the SQL ability so was used to set up the Zen Cart. It was never foreseen that forwarding would become a problem.

The forwarding of the first domain to the second is handled by a simple box on the control panel of its ISP, where the address of the target is filled in.

Move the primary domain to be hosted where you can run sql -php and ZenCart

I had feared that would be the suggestion! Because to move domains involves quite a lot of time, effort and cost, together with a considerable amount of downtime for the site - so I was hoping there would be some other workaround.

Interestingly, our first ZenCart (1.3) of many years ago (as mentioned in the opening question) worked OK with the same web forwarding, so presumably its been changes within ZenCart that stops it working now?

Also, earlier comments in this thread mentioned framesets as being a problem, and I've just discovered that the ISP looking after our primary domain has an option on the forwarding to use frames or not. So I've just changed that from using frames to not using frames ... so I'll soon see whether that makes any difference.

Update: switching to "no frames" on the forwarding seems to have worked! I will let it settle and do more tests, but on the face of it the problem is cured. Many thanks for everyone's help, and Happy Christmas too!

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Because to move domains involves quite a lot of time, effort and cost,

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Should only take about 4 hours