Thanks for your answers.
In fact, I forgot to mention that I tried to reconvert the ssa file to an srt file, and the result was the same...
I also tried to correct th errors with Winsubtitler, it didn't work either :(
Here's an example of sub which causes problems on BSpLayer:
http://py1378.free.fr/sub/LOST_1x23_Exodus_Part_1.srt
I will try Vladimir's Subsync...