funny, just saw, that the bug is quite exactly 10years old, which is why it looked like from this month. The problem still looks very similar but it isn't a regression from 5.4.0.89 to 5.4.0.91 at all. The kernel seems to get confused though if you mount a share once with and later withouth unix extensions. In such a situation unmounting *all* cifs shares helped to solve it. But it still looks buggy that area.
funny, just saw, that the bug is quite exactly 10years old, which is why it looked like from this month. The problem still looks very similar but it isn't a regression from 5.4.0.89 to 5.4.0.91 at all. The kernel seems to get confused though if you mount a share once with and later withouth unix extensions. In such a situation unmounting *all* cifs shares helped to solve it. But it still looks buggy that area.