Yellow Swordfish said

Blaise Fiedler said
I think the problem is gone. Mr Papa’s trick of deactivating and re-activating Simple Press worked. Well, I tried about 50 pages and did not get the 404 error, so I can’t conclude a 100% success, but still much better than the 50% success I had. 

OK – all this activity really does is rebuild the permalinks/rewrite rules once again

Interesting. But it seems to have done it in a better way than the rebuild button… I suspect Redis Object Cache has something to do about this.

Why  would it matter?  you said you are not caching the forum page?  if you cache it, you will get odd results since a cache assumes the page is static…  its not, its dynamic…  most object caches running at server level do a much better job and should be able to function… APC and memcache for example will work in my experience…  never heard of Redis…  its a php opcode object cache, should be fine….  but has to be low level server based…

Mr Papa said
Why  would it matter?  you said you are not caching the forum page?  if you cache it, you will get odd results since a cache assumes the page is static…  its not, its dynamic…  most object caches running at server level do a much better job and should be able to function… APC and memcache for example will work in my experience…  never heard of Redis…  its a php opcode object cache, should be fine….  but has to be low level server based…

I am not using a page cache on the forum, but I am using object cache everywhere. 

It matters, because it keeps some options in memory. WordPress heavily uses transients and object caching for a lot of its options and when you move to a persistent object caching such as Redis or Memcache you might end of having some options that are not refreshed / deleted when required. I did purge Redis several times when I was faced with the bug but only a full deactivating of SP closely preceded by a redis flushing really worked. 

so simple press does not use the caching components of wp…  if redis relies on that, performance may be sketchy at best…  but simple press does work with op code caches at the server level…  this redis sounds like a wp plugin vice a true object or op code cache at server level…