I've seen this many times on different versions, mostly when connecting via MAC and downloading plugins. Normally I just keep trying until it downloads, or connect by IP.
If you can't use IP (because it's not set), use Neighbor Viewer and terminal (link here) and connect to it by MAC terminal. Set up IP address using command line, and then use winbox normally (connecting to IP instead of MAC). Once it download routeros.dll and other files, you won't have problems connecting to any other routers that have V3rc10 anymore.
sometimes RB have default IP addres 192.168.88.1 to avoid problems with routeros.dll
you have to add IP address on your PC, for example 192.168.88.2
then, when you start winbox, you must change field CONNECT TO: xx:xx:xx:xx:xx
in CONNECT TO: 192.168.88.1
if you don't have default IP address you can use neibourhoodviewer.exe
and print > ip address add address=192.168.88.1/24 interface=ether1