Hi,
we tried to do an upgrade from 1.10.2 to 1.12.2 on our server, which resulted in many of the drawers turning into observer blocks and some drawers going missing.

Here's what we had before:
StorageDrawers-1.10.2-3.2.6
Chameleon-1.10-2.1.7
forge 1.10.2-12.18.1.2095

Here's what we tried to run:
StorageDrawers-1.12.1-5.3.3
Chameleon-1.12-4.1.3
forge-1.12.2-14.23.1.2555

(We also have these other mods: Bloodmoon, CustomMobSpawner, Dr Zharks MoCreatures, Pam's Harvest Craft. If you need to know more, we can provide the information.)

Do you have any ideas what could cause this behaviour and how to prevent it?

You may need to upgrade to 1.11 first to get the drawers to upgrade properly. I don't remember for sure though. I did try to preserve some upgrade path.

Hello again,
we tried that now. Unfortunately, MoCreatures has no 1.11 version, so an upgrade with 1.11 as an intermediate step doesn't work. Any other suggestions?