Yes, we are on version 4.6.10
We are just preparing the switch to the latest version ... But as it is a big site, we always do tests before changing version. Our tests are done on a server strictly identical to the production server (it is a mirror server updated every day). On this server we have a copy of the production site (so it's a test site with the "test site" license).
On the test site it is impossible to do the update, because the update process apparently makes a cache cleaning ... after which it is not possible to do anything.
Since yesterday, after some investigations, it seems that the problem comes from the "Friendly URLS" (if I totally deactivate the friendly URLS the site works correctly, even after emptying the cache, but then impossible to reactivate the "Friendly URLS" : the 1S160/2 error reappears).
As long as this problem persists on the test site, I can't consider trying to update the site in production 😞 , so I'm looking to see if this problem is known and if there is a solution before generating a support ticket.
Thank you for your help.