Hi!
I have made an upgrade from 2.x to 3.x and then directly 4.x. Unfortunately I get the following error message:
502 Bad Gateway nginx
I have asked for help a while ago in another thread: No Webinterface after Upgrade to 3.X - 502 Bad Gateway (I cleared my browser cache)
Unfortunately I couldn't solve the problem until now. So I don't have access to the web interface since end of february. So far I have helped myself by shutting down the server via command line and I did the weekly backups to the external usb backup drive manually. But I would love to be able to use the webinterface again. Just much more convenient... 
I ran an update / upgrade process today again. And during this process a few error messages appear when running update and upgrade.
Any idea on how to fix this? If I am not able to fix this, my only chance is probably setting up a new instance of OMV, but this would also mean that I would have to set up all users, all shares, etc. and I remember from last time, that this was quite a hassle.