Unfortunately, it took a while, but I found the error after searching.
A FLAG was accidentally used twice.
Furthermore, the units' reaction to wind has changed. As a result, the first target was moved out of the trigger zone, which led to completely illogical behavior.
I can't say which update this has changed with, but such a net positive adjustment can lead to unexpected and difficult-to-find problems in previously created missions.
The first objective is also leisurely to find due to a graphical error. I have also changed this and wish everyone good tactics and a successful search after the update in September.
CHPL's post in MISSION 3 / TIPS AND ISSUES was marked as the answer