Fred,

I see two possible situation:

1) The problem GFCI is defective and needs replacement.

2) That GFCI is protecting other downstream circuits and
is detecting some minor GF problems from downstream.
The espresso machine may then become the "straw
that breaks the camel's back." GFs are accumulative
and the GFCI is effectively summing all of the GFs on
circuits that it controls. Note that the GFs may be
real problems or just so-called "nuisance" GFs.

Somebody really needs to market a plug-in device which measures
the level of GF for any device plugged into it. If anybody is aware of
a reasonably priced device that does this, please let me know before
I start building one.

Such a device should be pretty easy to put together with a portable
GFCI, a 120V outlet splitter which makes it possible to plug 2 loads
into the portable GFCI, a potentiometer for inducing a variable and
measurable GF, a fixed value resistor to limit the induced GF level,
a multimeter for a one-time calibrating of the potentiometer, etc.

Fred, you probably should buy or borrow a GFCI tester which has
user selectable GF settings. MCM Electronics sells such devices
and I'm certain that there are many others who sell this item. This
makes it possible to test your problem GFCI and to determine what
level of GF plugged into that GFCI will trip it. You can perform the
same test for the other GFCI in your kitchen.

Also, remember that GFCI units are relatively inexpensive and very
easy to replace. I paid $4.49 each for the most recent batch that
I purchased. To simply replace the GFCI and see what happens
may be the most pragmatic approach.

Let us know what you discover. We all appreciate the feedback.

Good luck,
Gideon