I have bunch for reference in bib file.
Is it possible that when I type \cite, it will pop up a windonw listing all
the reference I can choose from?

that should already work ...

...how can I activate this function, if it dosn't work? In my Project, it did already work, but now this function ist not available an I don't know why?
I switched from biblatex to biber. Could this be the reason?

If it does not work, likely the bibliography is not found/loaded.

If you type in the command completely (\cite{aValidKey}) is the key marked as unknown?

One situation where this can happen is multifile documents. If you don't open the file defining the bibliography it is not known to TXS.

how does a unknown key look like? Is it a special color?
My document is a multifile document. I have a main-document and the chapters are seperated. I opened already all the files (icluding the bib-file) but the keylist was not available?
4 or 6 weeks abgo, it available...unsure looking..

Unknown keys have a wave underline. If you hover the mouse over them the tooltip says "Citation missing", otherwise it should display the citation info.

It's difficult to find the problem without a concrete example here. Could you please post a minimal example?

the cite is unknown. ...unfortunately ...???

I could send you my project per Mail, if this would be helpful. Tayloring an minimal example, is not so easy for me - I'm a beginner, so I don't know, what is necessary an what not?

well, I have do not much time at the moment. So my reply might take some time. But if you want, you can send the project to me.