Nope, but I found a news blog dealing with MTP-driver errors in Windows when I searched for the class ID {eec5ad98-8080-425f-922a-dabf3de3f69a} of the items in the device-manager which were marked there with that yellow triangle. The guy wrote that it could be connected to a faulty "UpperFilters"-entry and indeed - there was a value named "WpdUpFltr" for its UpperFilters-entry.
I backed up the branch HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{eec5ad98-8080-425f-922a-dabf3de3f69a} with regedit.exe and removed the UpperFilters-entry as in his screenshot this one was also not present.
I rebooted and after that the errors disappeared and I got my drive letters under the node "portable devices" in the device-manager again instead. Yay! 
What I don't know is if the change has any other implications, but for me the result looks good.
Case closed... for now.
P.S.: The USB card reader was working all the time, that's something I did not mention before... it was just this error-sign in the device-manager that was bugging me. I could connect USB-devices to the USB 2.0 and 3.0 ports and even SD-cards without any problems.
