Do you have any older saves that work properly?
Try deleting all of the files in the '
..\Divine Divinity\Run\Dynamic' folder.
This folder is used as a cache for the game. Files are created / copied there from the save game folder when you load, and written / copied from that folder when you save a game. The files will be re-created as required the next time you start Divinity. If there was a corrupt file in that folder, deleting the files should fix the problem.
If you start a new game, can you open the chest in the starting room, and take the ladder up? If not, then the problem is likely a corrupt file in the game folders (possibly the main items.000 file). In this case, a clean install of the game should help. Backup your savegames first (just in case), then uninstall the game, reboot, make sure there is nothing left in the Divine Divinity folder (other than the \run\savegames subfolder) and re-install the game.
If a new game works, but your current game does not, then the problem is most likely in the save itself. In that case it may be possible to fix the problem by replacing the corrupt file(s) with those from your most recent working save (if any), but that could cause other problems (possibly missing quest items, though there is a workaround for that) depending on the file(s) in question.
Troubleshooting Tips (& manual install procedure)Do you have the CD/DVD version of the game or the GOG/Impulse downloadable version?
Was Srimpo causing a crash before the problem with the stairs/chests/loot, or have you only tried that quest since then?
At least with the disk version of DD I don't recall a pattern of crashes with Shrimpo. How are you fighting him? Perhaps if you avoided weapons with Freeze or summoned creatures that could avoid the issue, or something...
If this problem still persists after you get the first one resolved, you could try using
DAD's item editor to give a very large damage value to some crappy weapon to use temporarily. A one-hit kill would avoid the 40% health point, so maybe it might bypass whatever is causing the crash.