3D Screen Saver causes error 380 with userform text box ControSour

I have found a strange phenomenon:
In my Excel VB project I set userform text box ControlSource property, which
normally goes fine. However, it may fail sometimes, giving the error 380:
"Could not set the ControlSource property. Invalid Property value." I spent a
lot of time to find out what causes this failure. It seems now that the
failure appears if a 3D Screen Saver has been connected on meanwhile. It is
possible to get rid of the failure by closing Excel and reopening it but it
is a nuisance.
Possibly this is a Screen Saver problem but I found it convenient to report
it here. Only 3D Screen Savers seem to cause this problem but swapping any
screen saver via Control Panel/Display and its Screen Saver tab fires the
problem as well.
I have Win XP Pro with SP3 and all Windows Updates for my Office 2002.