Frequent smbd crashes with smbd.core

Frequent SMB crashes which result in aborted file transfers. Clients are Windows desktops (7 & 10). I haven't been able to figure out if there's any particular activity that causes it, because it happens when I'm accessing files and also when nobody is accessing files (i.e., middle of the night).

System was upgraded from FreeNAS 11.x, and problem appeared immediately after upgrade to 12.0. Subsequent upgrades, up to 12.1U2, and problem persists. There have been a few similar reports here of smb issues, but I haven't seen a resolution, and they often appear to be related to other issues that I don't have (problems with ssh, syslog, time machine, etc).

Core and debug will be attached shortly.