OK, did some more investigations and surprisingly I see the same blue taxi lights. With the help of our dev Stefan, who has of course a vanilla P3D installation, we found that indeed the dev of another addon simply replaced the original file with his own version, which results in what you/we see:
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The file in questions is fx_2.bmp
This stupid dev did something that is not allowed by LM. If a dev needs his own effects file, he has to give it a another na