When i call the script, the group is created, but after a few seconds all but 1 guy disappears
Also, after the disappear, i can walk toward the center of the trigger and they respawn again.
This is a wanted behaviour. Whenever a group is created (not implemented for vehicles) a secondary script is launched for each group, checking distance to nearest enemy of the group. If teh distance exceeds a certain range, the group is reduced to group leader and the rest is stored. Whenever an enemy closes up, the group is restored and should behave as they were there all the time. The group leader always remains to guarantee realistic group movements while enemies are away. This way you might guess where the group could be whenever you return to them. No fancy random replacement.
This feature should help to reduce CPU load as not a group of maybe 10 units has to be calculated but only a single unit. Especially in bigger missions, human players might change theyr mind and after a first attack maybe switch to another objective. As the zone is already activated at this point, switching to another objective and activating more zones might lead to CPU slowdowns.
About the error in the arma.rpt: never seen this before. If nothing else goes wrong and everything works fine, you may ignore it. If it bothers you or it doesn't work as expected, please send me the mission as editor version so i can take a look at it.