On some VE edits, when I click 'Review Changes' or switch to 'Source Editing', the request goes through just fine. But, on some requests, I get the error / page hangs.

Unfortunately, I couldn't find any Parsoid-PHP log entries for officewiki. But, when I expanded my search to all logstash events, I found https://logstash.wikimedia.org/goto/bedbf8dd6b360e16945d63c0a9df13f7

So, a number of ApiParsoidTrait::transformHTML: Received funny ETag from client: {etag} and ApiParsoidTrait::requestRestbase: Received HTTP {code} from RESTBase which are the http 504s. All errors are for transform/html/to/wikitext/Community_Relations_Specialists%2FAudiences%2FMeetings_with_Product/$OLDID.

Do @Pchelolo or @matmarex know what these are about?

There are a couple of hits on 2020-08-10. Looks like the etag is the empty string (or null or undefined, I don't know exactly how those are being stringified).

EDIT: indeed, I suspect 'null' is indistinguishable from the empty string here.

There are instances of this message on other wikis as well:
https://logstash.wikimedia.org/goto/8165f7ca859f7897f4fed689b2cd3a48

This was probably the same problem as T244609 (because the page causing issues here is huge like in that task, and it's the same error message).