First of all, update your game client to 5.43 - it's been out for like a month now and any older client will stop working relatively soon.
If it still happens after the update, make sure that

• You have proper write/read permission on your Toribash folder in general and customs folder specifically
• Your customs folder nor your item.dat is set to 'read-only'
• You don't have any software running that may be interfering with Toribash's networking

If even after doing all of the above you still have the same issue, run the game to replicate it, close it and upload the stderr.txt file here (it's generated on game exit inside your Toribash folder).