I have tested A3 without any mods or addons enabled and the above mentioned bugs ceased to exist - everything seems to work like its supposed to be then. After that, I selectively enabled all of the disabled mods/addons again until these bugs reappeared again. By this way, I could finally pinpoint the troublemaker: it's the C2 - Command & Control AI mod! I could reproduce the disappearance and reappearance of the above mentioned bugs by disabling and re-enabling the C2 mod!