so, this might be hard for you without a german keyboard layout but we just did run into this issue again:
One person has changed his password yesterday and made it in a way that the character "ä" was in it, after this change he wasn't able to log into the ROC.
It took us quite some time to get to the root of this problem as he was able to log into automation studio!!
It seems that the login is treated differently in the java application as it is in the release operation center, so it would be nice, if both work kinda the same way.
We currently have a list of the following characters that should not be used (we assumed "ü" and "ö" won't work as well if "ä" doesn't): ä, ü, ö, ß and §
although we still have no idea, why the paragraph doesn't work. But we have an open case for that with CA: 208526 (seems like the "core team" is still working on it)
I'm just writing this now in the community to inform others of this issue.