The customer called, complaining that he could not decently browse his brand new FFCore 5.4.4 (he used to operate a 188.8.131.52 beforehand): the list of Workflows is left empty (whereas he has dozens of them), "Submit Job" is blind, etc. Chrome works like a charm.
The first reflex here was to dive into the 5.4 Release Note, which states that "FreeFlow Core supports current versions of modern browsers from Apple®, Google®, Microsoft® and Mozilla®".
Good to know! Now, without arguing, is IE11 a "modern browser from Microsoft®" ? Certainly not... :-|