Thanks again for your efforts, swfan28.
In the meantime I've studied some aspects of Kotor modding (phew) and I'm pretty certain, that the Weapons of the Republic - Wotor mod is the reason for this whole mess. It seems that my new save games have all been corrupted in some way, since the old ones without Wotor items are still functioning. There are mistakes in the Wotor utc files as well, therefor I will delete every file which uses utc or scripts for the main characters Malak and Bastila; too bad I didn't see the bugs earlier.
BTW: I've compiled a script k_psta_bastrom.ncs which is needed to get those "special" dialogues with Bastila on the Star Forge, if you are using Bastila and female Revan - untold love story mod and Allronix' dialogue pack (k_sta_bastila.dlg) together...no gender check anymore, it's just the Bastila dialogue from now on.