I stand corrected, it was indeed a DVD+RW. The subtitles aren't displayed on my standalone DVD player, and I just found a pattern. It seems that if the filename (both srt and avi) is over 53 chars then it truncateds its name and won't show the subtitle. It did work on my PC, so I guess it's a problem with my standalone not ImgBurn.
Anyway, somehow Nero handles it differently, because when I burn those same files in Nero the sub appears even on my standalone.
Thanks for your help though, I guess I will start renaming long filenames.