prep failures during TCH assignment are usually caused by faulty BTS/BSC hardware, as you mentionned, or a problem of in the NSS.
have you changed BSS release or NSS release (or path ?) lately ?
if it happens to only one BSC, i’m keen to think it might be a problem in the BSC itself. Try to isolate exactly which cells have this problem : all cells of the BSC, or just the cells that share one specific TSU ?
No omcr alarms doesn’t mean there is no problem with the board.
Once you have isolated the problem, perform some A interface or Air interface traces, to see where exactly is occuring the issue (during which message ?). Then raise the issue to TAC 🙂