I've also been using XLD for years, and recently I am frequently getting an "error has occurred in decoder" message. I would have thought this was due to corrupt files, but now I'm on my 3rd set of FLAC files (originally live recordings) which have some "error" FLAC files when converting to MP3 V0 with LAME. XLD apparently has no log. I tried to convert to WAV with xACT, but also get errors on the same files.
I tried reselecting output directory as suggested here with no effect (source FLAC and output on different volumes). I tried copying the FLAC files to the same volume as intended output, no result.
It seems like a big coincidence that suddenly I'm getting lots of "corrupt" FLAC files. Is there a problem with XLD and OS X El Capitan? Or is there a way to fix these FLAC files with another tool?