As mentioned earlier, I am absolutely NOT sure that it is the update itself that is the reason why my customer login no longer works.
It's just after the update that I have been made aware that it no longer works.
I have a test account, which I myself have used by March 4 without any problems, so at that time it worked without any problems.
I do not remember making any other changes since then, which is why I initially thought it was the update that was causing the problem.
Now I suspect more closely the theme I use (IQIT warehouse). Maybe even a combination of the update and the theme.