Google Tag Manager Server-Side preview not working

Question

Over the past two weeks, I've discovered preview-mode no longer works on new GTM server-side deployments.

(I'm no stranger to setting up and configuring google tag manager browser and server. I've deployed MANY of these and implemented highly customized setups.)

I'm automatically provisioning Google Tag Manager server-side on GCP App Engine Flexible instance. Nothing custom.

Paid for GCP support. They said to get in contact with the GTM team or the community support group.

Tried lodging a support request on the Google Tag Manager community support group. No answer.

Can't find a way to contact the GTM development team.

To investigate, I tried pairing GTM browser with older GTM server setups I deployed months ago via the GA4 transport url method. Works without problem.

I narrowed the issue to some kind of server-related issue:

Debug memo dispatch to http://localhost:8081 failed: 
Error: cacheableLookup ENOTFOUND localhost at CacheableLookup.lookupAsync (/workspace/node_modules/cacheable-lookup/source/index.js:199:18) 
{code: 'ENOTFOUND', hostname: 'localhost'}

Answer 1

This is a known bug, has been diagnosed by the GTM team, and will hopefully be fixed very soon. The problem is that App Engine changed that localhost no longer responds to the requests from the tagging server, so Preview mode is unable to show the results.