I have the same darn problem and it just started towards the tail end of a (what I consider to be) relatively small project.
There are maybe 12 to 14 audio tracks, most mono and 3 instrument tracks. The instruments are Halion, Kontakt 5 and Superior drummer.
I have hunted through the *.log file and as it always is, was greeted with that gobbledygook that only Steinberg can decipher.
I had to edit some vocal material and NEEDED the copy feature (by the way, it does the same thing no matter what copy method I use. ie toolbar or ctl c) so the only thing I could think of was to “duplicate” the track I wanted copied and slide the wav form of the duplicate around placing bits and pieces of it where I needed to. It wasn’t too time consuming once I cut the track up into various sections of the vocal performance.
I hope it is a song specific issue and not one I will face in other projects I am currently working on.
Thanks for posting this, it’s nice to know I am not the only one! (sorry to imply I am happy you are having issues as well, that’s not what I meant…I am hopeful you know that)
Mark