Thank you for your reply! However, my standalone players (2 Pioneers) play DVD-R / DVD+R normally in spite of the use manual states that they're compatible only with DVD-R. But I have over 30 DVD+R and they play fine.
In this meantime, I contacted the technical support of Alcohol 120% (by the way I had an excellent support in spite of not to be an Alcohol 120% user). I was told that, in order to make DVDs more compatible with standalone players (with different firmwares), the media must be burnt as DVD-ROM and this option must be selected previously, before to burn the blank disc (obviously).
Summing it up: it seems that there is nothing to do with my current disc (which plays on my LG drive but not on my standalone players). For the next discs, I have already asked my contact to burn the media as DVD-ROM, which, by the way, he wasn't aware of.
Hope the forum readers may use this small trick got from Alcohol 120% (excellent!) support. Thanks again for your kind reply.