Starting with the CC 2015.3 release of Premiere Pro, the old Tesla-architecture GPUs (of which your GTS 250 was one) are now officially obsolete. In fact, the GPU that the GTS 250 used dates all the way back to the 8800 GT 512MB from way back in 2007! That's almost 10 years old! And there are no new driver patches for that series of GPUs from Nvidia since April of last year – in other words, Nvidia had already EOSL'd ("End Of Support Life'd") the entire GeForce 200 series GPUs for almost a year now.
As a result, they are now no longer supported by Adobe for CUDA MPE GPU acceleration. Therefore, the only fix is to concede that you'll be permanently locked to the MPE software-only mode for as long as you continue to use that GTS 250, and buy a newer GPU such as a GTX 1050 Ti if you want to use CUDA MPE GPU acceleration.