I don't get 404 here

Each one came through fine for me on several browsers, several times with no 404's

Perhaps a google issue?

Hello (again);
My soft 404 error count has now been reduced from 41 to 10.
Through the Google Search console, those 10 are requested to be re-indexed. The next day I have 10 more soft 404 errors, and I am daily requesting re-indexing with the new 10.
It would appear that this won't stop until Google has re-indexed every page.

Sounds like your ISP/Host may be having some load/demand/resource issues occasionally.

I have over 300k photos in my collection with no degradation in performance.

I'd have your host look at their logs and utilization in an effort to assist.

Sorry there isn't anything else I can offer, as it would be host and the logs for SQL and Web Services that would be the indicators for any 404 errors.  (and or potential traffic blocks - but not likely on an individual account unless you chose to throttle)

Hello;
I believe that I have resolved the actual cause of my apparent performance issues and why was I getting some odd errors and the spurious soft 404 errors? These were slowly going away as I asked Google to re-index them? Nearly all were from images held within the Piwigo photo albums. As you know from earlier messages it was difficult to shed any light on the cause and were suggesting that there may be some processing hold-up within my ISP's servers.

However, I was periodically generating new Google xml site maps and uploading them to my website, expecting Google to find them and slowly work its way through the latest version.

Alas, this was not the case.

Meanwhile, I had some Google errors that I couldn't fathom out, such as no viewport set when that was not the case. Why I wondered was this and why hadn't Google found them in the latest copies on my site?
Also, being an ex-computer manager and consultant, I had always wondered how Google indexed each site, where was the processing done, when was it done; overnight or at some other time without impacting on my ISP's server's (34sp in my case) ability to respond to requests? All the while only part of my site was being indexed.

I know it takes a while for Google to work its way through a site so I was not unduly concerned by this, but was wondering when it would complete it?

By accident all became clear. I was browsing through Google's new Search Console when I finally checked in on Sitemaps. The recorded entry was 3 years old! Even though I had updated my site many times and the xml sitemap too, I didn't know (and presumably neither do a lot of other people) that I also had to manually record the latest generation within Google, believing that all I was doing was making Google aware that one existed at the time I created my first one. The one that I had originally recorded had 2000 image references but my latest had 8800. This is where I then began to understand how Google indexed and why I had spurious errors.

Each time I generated a new xml site map even though it was uploaded it was not recorded within Google. Google sort-of took a half look at it. But the real crux is that Google's indexing must take place on its own servers, so there is no performance degradation at each host's site. Based on my experience, I believe that when recording the latest xml site map Google takes a copy of each site and slowly works its way through it as it indexed it on its servers. As my recorded site map was 3 years old it was basing its result, errors etc. on an older copy and size of my own site. So, the soft 404 errors are caused by new images being present, that Google finds, but are not in the recorded copy of the sitemap or of my site as of 3 years ago.

So, it sort of tells you something is wrong but without being explicit about the cause.
So, the solution I believe is a simple one, each time a new site map is generated, inform Google that it exists and delete the previous version. Then it can base its indexing etc. on the latest data.

I trust that is not too onerous to read and that it helps you to understand the causes and solution to this type of problem?
Is the latest version of the xml sitemap the one that is recorded within the search console, and if not make it so.