I was also told they couldn't print or preview an Inventory register, but I never saw that one.

After much troubleshooting we finally isolated it to Windows Defender, and needed to set up an exclusion to the MAS90 directory - problem solved. Sage 100c v2021 Advanced running on Windows Server 2019 (virtual).

Some interesting nuances to this issue:

• Didn't happen in MAS90 Mode
• Daily Transaction Registers in AP and IM previewed fine
• Happened in multiple companies
• 2 Library Master reports also generated Error #0s - although they had Info buttons (Program SY_Session.pvc Line 6689 & SY_Launcher_ui.pvc at Line 2585)
• Of course this server had been running with Windows Defender and Malwarebytes just fine for months before today

According to the IT guy, this is the likely culprit:
KB5011503 for Windows Server 2019 – March Security Update