FFMpeg - H264 video decoding with warning: SEI type truncated; error concealment; No accelerated color space conversion

Question

when calling avcodec_decode_video2 (pCodecCtx, pFrame, & get_picture, & package); to decode H264 video from a PMP file.

I often get the following warnings as:

FF: SEI type 1 size 40 truncated at 36 FF: error while decoding MB 23 15, bytestream (td) FF: Cannot use next picture in error concealment ... FF: No accelerated colorspace conversion found from yuv420p to rgb24. .... 

So far, got_picture still returns 1, but the video quality is poor, often vague and flickering. What is the problem? so what can I do? Thanks!