I experience the same issue. All I could see as well was some sort of Bus error.
Also on 2.6 IMG on the Pi 2. Saving appears to work fine – I don’t see any message about it ingame, but I see the files that are created (in my case on a mounted NFS share so I can continue to play in another room on another device).
I was thinking about permissions of the savestates…but they are fine imo.
I just succeeded in this problem.
I just made a complete binary update yesterday and, after completely reconfiguring my controllers for mupen64plus, I can now successfully save and load. Maybe give that a try. Takes a few hours, but totally worth it.
Oh, and maybe back up your mupen64 .cfg and input-file before you do that.