We appear to have jumped from teething troubles to problems caused by wear and tear with nothing in between. That can't be good.
I doubt that it's the door drives themselves that are at fault - they are an off-the-shelf module made by IFE used by multiple rollibg stock manufacturers in everything from trams to inter-urban trains. The fact that the door issues are mostly seen in the end carriages may be important. It was decided to only fit one set of doors in the longer end carriages (the Class 555s have two, as do most of the other Stadler METRO family designs). This will naturally cause passengers to congregate there under peak loading conditions leading to the floor loading problems that then cause the step mechanism to jam, which then requires the door to be isolated to allow the train to continue to run.
I was interested to read the post from last week about a 777 door failure on the Wirral where one of the doors came out of it's track. That could infer some degree of flexing or distortion of the bodyshell, which may also be a factor in the issues with the floor in that area. Under peak loading conditions, the passenger loading around the single door may be forming a large point load on the monocoque bodyshell, positioned almost directly at the midpoint between the two bogie pivots (like loading a fixed beam at the midpoint between it's end supports) where you would expect to see maximum deflection.
I wonder if Stadler did a physical load test on an instrumented bodyshell at the factory before going into full production, or did they rely on FEA simulations from the design stage and operational experience of previous METRO variants? It's not like they would be the first rolling stock maufacturer to discover unpredicted structural issues with their aluminium monocoque bodyshell designs once they had got into service.
