You can read the indices because they are cached. You cannot read the thread pages because they are not. This is typical of on-the-fly html. A couple of weeks ago PPRuNe was apparently switched to a php server.
There was already a discussion of this which you can see by showing topics from the last ten days.
I am not so sure that the remedy discussed there is needed any longer. I was in the process of converting a newsgroup archiver that I wrote to a PPRuNe archiver. But with the switch I stopped doing that because there are no longer embedded html tags, especially no nonlocal anchors, in the thread pages.
Try the following. Visit a thread page. Save the frame as something. Your browser will probably suggest ulimatebb.php. Save it as *.txt where you may use any name instead of *. Because it is not cached your browser will retrieve the file a second time in order to do the save. Then use any text editor to open up for reading the saved file. EditPad Lite is free for noncommercial use.
<a href="http://www.editpadpro.com/editpadlite.html" target="_blank">http://www.editpadpro.com/editpadlite.html</a>
I have found that almost all pages are quite readable this way, unlike the old cgi pages. (There are no formatting elements so don't expect it to look like a web page - just pure text). Dispose of the saved text files that you do not want to keep because they will not be automatically discarded like cached files.
I'm not sure if this will be close enough to what you want but I can't think of an alternative.