More information:

– I had W3TC installed and then removed about 2 years ago… this perhaps explains the warning message I get?

– I tried disabling Zencache, and this did not solve the error

– I tried enabling SP Error Logging, but I do not know where SP saves its error log.

Let’s do the W3 Total Cache message first. If, as you say, you had it installed and removed it then it may depend on how you removed it. If you deactivated it in the WP plugin listing and even deleted it from there then it is probably not entirely eradicated. WP Total Cache (and WP Super cache in fact) are not completely removed when deactivated. Both plugins come with special instructions on how to remove.

Simple:Press has checked for WP Super Cache for a while and we decided to add this new check for 5.6. It looks in the server location:

/wp-content/cache/config/master.php

and that master,php file was put there by W3 Total Cache – as was the ‘cache’ folder itself. So  if that exists in your server hence the message I am afraid. You need to remove these remnants manually.

Now on to the 404. If nothing else has changed then the only thing that comes to mind is that something other than SP has disturbed your permalink database record (rewrite rules), Try this:

1. Go to the WP Admin > Settings > Permalinks page and click on ‘Save Changes’.
2. Go to the SP Admin > Integration > Page and Permalink and check the correct page is displayed at the top.
3. If NOT the correct page then select the correct one and update the whole admin panel at the bottom. If correct continue with 4.
4. Click on the ‘Update Forum Permalinks’ button half way down the panel.

Hopefully that will clear things.

Great: thank you sincerely for the quick answer.

I already fixed the W3TC “problem” following your instructions. Indeed there was the folder there. Perhaps you should / could add a check on whether the plugin is activated as well in future releases. 

Checking the rest now.

Yellow Swordfish said

Now on to the 404. If nothing else has changed then the only thing that comes to mind is that something other than SP has disturbed your permalink database record (rewrite rules), Try this:

1. Go to the WP Admin > Settings > Permalinks page and click on ‘Save Changes’.

Bingo! this did it! Interesting, as I just clicked on Save Changes without changing anything 🙂 

For your records and so that in the future you can avoid this problem, here is exactly what I did:

– upgrade SP to 5.6

– upgrade the database

– upgrade Reboot + default theme

– upgrade about a dozen SP plugins

Nothing else. 

Interestingly, I did before on my test platform without zencache installed and it worked first time. In production, with Zencache installed I got the error mentionned above. Even deleting zencache’s cache did not fix the problem. 

Perhaps you can implement an auto save of the permalink after an upgrade?

Anyway, thanks for the support!

W3 Total Cache: I prefer to check for that actual config file. People do not realise that the cached records can still be active for a while after deactivation of the plugin. Some people may also deactivate while performing upgrades or maintenance. By looking for the config file – orphaned or not – it ensures that things get cleaned up properly as has just been the case with yourself…

We do update the forum permalinks at the end of an upgrade. This suggests that your WP base rewrite rules had become unstable which we do not touch. Under these conditions, updating the forum specific permalinks can have the result you witnessed. Hence updating the WP permalinks as a first step. If it happens again I would take a look at any other plugins you may have active that also create permalinks (if any) in case they are not adding correctly or flushing when they should not be.