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The problem is relatively simple. The .php files which are in the "preloads" folder are loaded automatically by xoops. When we moved from index.html to index.php, we modified core (2.5.11) to not include the index.php file. But on version 2.5.10 there is no this modification.
Including the index.php file generates 404 errors (not in navigation) but on robots or internal scripts on the site.
Several weeks ago, I discovered that one of my websites was no longer referenced by google, the robots were blocked (error 404). I searched (very long) and found this problem. It is for this reason that all modules are modified with index.html in the "preloads" folder.
On xoops 2.5.11 there is no problem but before yes...