Attachments loading issues (500 Errors)

With reference to this post: https://www.phpbb.com/community/viewtopic.php?f=466&t=2282631&start=30#p13858056

I have just created a fresh 3.1.2 install. Created a topic with attachments and for some reason all of them will never show at once. But it is a little sporadic, if the page is refreshed then some load, some don't, sometimes ones that did load now don't load. The ones that don't load give 500 errors.

There is a test link within the phppBB post above where it can be seen. (I'll upload the pic/site address here too as I don't want to put the link online) This seems to be an ongoing problem for me and I'm posting it here just to bring further attention to it, in case it's a definite issue with the software, or to confirm that it's a specific server etc setup that causes the issue.

My live site depends heavily on attachments, and I've noticed that if there are enough posted that some users avatars fail to load also. I've tried my best to resolve it, but I'm stumped.