unpredictable acad crash caused by editor-reactor

I have a weird problem with the command-ended event of an
editor-reactor. When just one drawing file is open, everythingh runs
smooth and fine. But when another drawing is opened, the command-ended
reactor function sometimes causes AutoCAd to crash. This behavior is
not reproducable - one and the same drawing at one time can cause the
crash, and at another time will open just as it is expected to do.

I modified the reactor function so that it does nothing when the
calling command is "OPEN", but this made no difference.

No I found that when I let the reactor function (print "OK") there is
no crash at all. Funny that (print 'OK) is no help

Instead (print "OK") I can also have the reactor function create an
entity (line or something else) and entdel it immediately again, and
this also prevents AutoCAD from crashing on OPEN.

Does anybody have an idea what's going on here?

TIA
Tom Berger