I might have same problem, only on my Sandberg TM2. Only happens when I am using dunlop super bright string (was buy one get one free, so I bought 5 sets), it sounds like dead string. But when I put roto string on it, the problem gone, back to another set of dunlop, same sh*t happen.
There was a video (by overwater?) says if u got fender-ish bass, do not cut A string when changing string, but doesn't work on my Sandberg.
Just my story, hope someone can explain this, I don't play my sandberg much but this made me crazy.