Hi, Klause,
this is hardware issue. Windows itself does not know what memory is installed and cannot look at it so Windows ask memory modules to self-identify. If there is no correct answer from memory, Windows says "unknown". Memory modules are connected thru memory controller (it is located in CPU now nowadays) so minor inaccuracies in interactions may cause such "unknown" message. Same inaccuracies may prevent other programs from detecting RAM modules.
This does not affect real memory working mode.
I hope I've understood your question correctly. If not - please clarify.