When order now is clicked, the page keeps on loading and the order status page is not shown
The issue is caused due to the MID server that is incompatible with London version.
The issue is caused due to the run script activity in the workflow that is attached to the catalog item.
The Run script activity is calling a RESTMessageV2() web service, that uses the MID server.
This MID server is not compatible with London version, so it is down.
Due to this reason this Run script activity will be stuck in Executing as there is no MID Server to pick the RESTMessageV2() call.
Therefore once the item is submitted and the page keeps loading until the transaction eventually gets timed out.