Thank you for your advice, jesusalva.
I played around a bit and - finally - I managed to work around the bug. But I
don't understand clearly why.

As a first attempt I looked up the gate labels in the level editor. They seemed
to be set correctly (they also appeared in the map/levels.dat).
I thought, the error must be caused by a wrong obstancle ID. The error complaied
about a missing obstacle ID '132' (resp. '66') which isn't listed in
map/script_helpers.lua. So I simply added two list elements with ID '66' and
'132' with an "opened"/"closed" pair. I don't know what the status IDs mean, so
I tried some out.
=> The terminal dialog worked correct that way, but the gates still didn't open.
Instead some strange things happened - walls next to the gates turned into open
gates etc. So this wasn't the right approach.

As a second attempt I followed Jesus' advice. First I deleted the gate labels
and tried out what happened in this configuration:
=> The game crashed when I clicked the "-status" action in the gate access
terminal. This message was printed in the console:
---------------------------------------------------------------------------------
FreedroidRPG 0.15.1 encountered a problem in function: 
give_pointer_to_obstacle_with_label 
Obstacle label "HF-Gate-outer" was not found on the map.
...
---------------------------------------------------------------------------------
Then I re-planted the gate labels. When diffing the map/levels.dat with the
original version it seemes to be the same as before (only the two label entries
appear some lines below their original position). But ...
=> This configuration now works perfectly (terminal dialog works + gates get
opened)!

If anyone wants some of my files for further examination, just let me know.

The sad thing is, I don't know how to transfer the change to my active hero's
shp file. I renamed the MapEd.shp for test.

Cheers, Ferris