SecuExtender suddenly not able to connect to USG40

we use since a while a secuextender to connect from a PC, through the internet, to a USG 40.
Today it stopped working:
if one tries to connect, it says like this:

1) after pushing the connect button, it appears (as always) a window saying "this connection is untrusted" (it is a matter of security certificate, that I did not manage to solve yet) - while the symbol on the taskbar turns yellow.

2) after saying "yes" (do you want to continue...?), the little symbol on the taskbar (next to the clock, in windows) remains red, and says "secuExtender connection is disconnected".

I am using the last secuextender (4.0.2) on win 7.
the USG40 seem otherwise to be still reachable, over the outside https address (so it is not down or so...)

Does anybody has an idea why this could happen, or how can I debug it?
Thanks a lot in advance!