M. balfouri bad moult :/

[Nonnack]

I had this M. balfouri since first instar sling, for about one year. It had about 7 moults and now this happened. I noticed that it is in same position for 3 days, so I bend container slightly and it didn't react, after few days I finally decided to dig it up. It looks like it stuck in moult just the carapace and abdomen get out,all legs are still stuck. Ofc spider is dead. Humidity was ok I suppose, especially that I raised it a little bit expecting moult. I have also spotted a small pool of something under spider, probably lymph. Any ideas what was wrong, or how to avoid things like that?