So I upgraded to OMV4, and it went pretty well: my ssh connection dropped during the upgrade process, but a dpkg reconfigure and apt upgrade later everything is running smoothly.
However, the update's broken SMB for me. So far I've verified that it is indeed the upgrade and SMB that caused this, because the problem occurs on both my desktop and my Android phone. /etc/samba/smb.conf looks fine, and FTP works well enough. I tried forcing SMB2 and SMB3 from my Linux box's end, but it didn't work.
What logs or other information do you guys need?
EDIT: Turns out it just needed more apt upgrades.