I have the 1.21RC1 firmware, but i had the problem with the 1.20 also
When i "uppload", or put "directly" a big gcode ( 64mb) to the sd card i get kicked out of the webgui. ( and when i reconnect i see that gcode is "loading" the gcode. And it is a * in the beginning of the filename. and get kicked out again.
I have to delete the gcode from the sd card to get into the webgui, so i not are beeing kicked out.
What can this be ?