i've found the cause of these issues.
Its usually cause by outdated steamcmd.dll's which makes the workshop download fail.
Just replace the dll's in your PZserver install directory with the "fresh" ones of your steamcmd root folder.
source:
https://www.reddit.com/r/projectzomboid/comments/1lyw5ww/psa_for_all_server_owners_who_is_facing_steam_mod/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1

Server won't restart.

LOG : General , 1752713337626> 1,353,468> Workshop: onItemNotDownloaded itemID=3100657286 result=2
LOG : General , 1752713337626> 1,353,469> Workshop: item state DownloadPending -> Fail ID=3100657286
src\tier0\threadtools.cpp (3465) : Assertion Failed: Illegal termination of worker thread 'CFileWriterThread'
src\tier0\threadtools.cpp (3465) : Assertion Failed: Illegal termination of worker thread 'CFileWriterThread'
src\tier0\threadtools.cpp (3465) : Assertion Failed: Illegal termination of worker thread 'Thread(0x00007FFD7B601B60/0x000'
src\tier0\threadtools.cpp (3465) : Assertion Failed: Illegal termination of worker thread 'Thread(0x00007FFD7B601B60/0x000'

Same here

Workshop: item state DownloadPending -> Fail ID=3100657286