Another day, another issue to resolve… this time it was the turn of phpBB to shed some of the remaining hair from my head. I have however now got to the bottom of this, so this is another post aimed at helping others hindered by this irritating issue.
It seems that the phpBB update changed the owner of the user uploaded avatars to my own id. This meant that phpBB was unable to access these in their current state (often using user 99 (nobody) to access things). I therefore changed the permissions of all files in /images/avatars/upload to CHMOD 644 (so all can read the files). This has solved the issue, allowing anybody (and “nobody”) to access the files. I’ve also had no issue changing an avatar since, this appears to be totally resolved by this process.
Hope that saves someone some stress!
For info, more on CHMOD settings for phpBB can be found here: http://www.phpbb.com/kb/article/chmod-permissions/