Hi there, I understand your problem. Volvo has a emergency release thing
under the side-covers in the boot. On my wagon, I get to it under the
lowest cover for changing bulbs. Its just a long string that you pull
and it unlocks the cover.
You might want to take off the entire cover on the side and pull out the
small electrical motor and lube it up with WD-40 or something. I did
that 18 months ago and it still works fine :)

I have a 93 850 that had same problem.

With mine, I couldn't get anything to work so I eventually got a hack saw
and carefully cut the latch (inside the panel..the little hook which is part
of the panel cover.  It looks OK but no longer locks so I bought a locking
cap.

I think what happened with mine is it got stuck at the clasp area...you
know...the way a door will not open if someone pushes from inside.

After I cut mine everything seemed to be functioning fine again.  What I
would suggest you do is push in on the access panel when you use the remote
opener and it will likely eventually open.

I'm fond of volvo's, currently have 3 and have had many more in the past:
loved em all but this 850 has really turned out to be a piece of junk.  Many
things break: transmission, power seat, air compressor, head reconditioned,
etc, etc.  I only drive it summers so the body is mint.

Good luck.

alan