TYPO3 Core

Using TYPO3 12.4.19. The strangest thing is happening with the Filepicker (TCA type = file) : When the modal window (aka Element Browser) is displayed and the user clicks on a filename to create a relation to that file, the filepicker is closing without making the relation. No error is thrown anywhere. Neither in the browser's debug console nor the server logs. Moreover, the live Apache logs show that NO QUERY is transmitted to the server upon that event/click (whereas it should send a query like so:

POST /typo3/ajax/file/reference/create?...

The problem is intermittent and I am still unable to find a pattern leading to this strange behavior…. Problem is happening with different browsers… No mod_sec rule is in the way.

The problem appeared when we upgraded our installation to branch 12.