B4J Question Error CS0016 creating standalone app

[LineCutter]

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Running: C:\WINDOWS\Microsoft.NET\Framework64\v4.0.30319\csc.exe
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Microsoft (R) Visual C# Compiler version 4.8.3752.0
for C# 5
Copyright (C) Microsoft Corporation. All rights reserved.
This compiler is provided as part of the Microsoft (R) .NET Framework, but only supports language versions up to C# 5, which is no longer the latest version. For compilers that support newer versions of the C# programming language, see http://go.microsoft.com/fwlink/?LinkID=533240
error CS0016: Could not write to output file 'c:\Users\Andy\Documents\B4J\ReadToList\Objects\temp\build\ReadToList.exe' -- 'Access is denied. '

Has anyone come across this?
No problems in the log up to this point.
I thought that it might be a Bitdefender problem (after an update), but disabling that doesn't help.
Java is up to date 11 JDK. I updated to the latest B4J, which might be when things broke, but installing the previous version over the top doesn't fix it.
The debug compilation (F5) runs perfectly & in fact I have been able to create an exe in the past for exactly this project.

Windows 10 (18363) 64 bit

 

[Erel]

It does look like an issue with an over aggressive anti virus.