https://www.truenas.com/docs/hub/intro/release-notes/12.0u2/

For those using TrueNAS with XP and below operating systems. If you are currently using U1 make sure you upgrade to U2 otherwise you'll have issues accessing the shares on your NAS. I usually wait a bit before installing the latest updates so that the guinea pigs can get bit....didn't help me here......
Took me quite a bit of time to figure out these symptoms:
Errors about being disconnected from the share
Windows accessing the share would disappear? WTF
Errors about how the share didn't support LFN
Errors copying files to and from shares