cfchart issues in a security sandbox on ColdFusion 2021 Developer Edition

A machine running ColdFusion 2021 Developer Edition has sandbox security enabled and the ColdFusion Lockdown Tool run. The following sandbox File/Dirs permissions have been granted:

• %cfusion-root%/tmpCache/CFFileServlet/_cf_chart      Read,Write,Delete
• %cfusion-root%/tmpCache/CFFileServlet/_cf_chart/*   Read,Write,Delete

A chart created using cfchart is causing a NullPointerException to be thrown (stack trace shown below), seemingly in the method call setUpWatermark().

Activating the server or disabling the sandbox causes the cfchart image to be displayed. In the latter case, the Not For Production Use watermark is shown.

Does anyone know what changes are required (perhaps additional sandbox permissions?) so the cfchart-generated image is displayed when sandbox security is enabled on a machine running the Developer Edition?

Thanks in advance.

java.lang.NullPointerException
at coldfusion.graph.InteractiveChart.setUpWatermark(InteractiveChart.java:328)
at coldfusion.graph.InteractiveChart.mergeJSON(InteractiveChart.java:452)
at coldfusion.tagext.io.ChartTag.doEndTag(ChartTag.java:1184)
at cfpathobfuscated2ecfm1167273304._factor3(path-obfuscated.cfm:106)
at cfpathobfuscated2ecfm1167273304._factor7(path-obfuscated.cfm:101)
at cfpathobfuscated2ecfm1167273304.runPage(path-obfuscated.cfm:1)
at coldfusion.runtime.CfJspPage.invoke(CfJspPage.java:257)
at coldfusion.tagext.lang.IncludeTag.handlePageInvoke(IncludeTag.java:749)
at coldfusion.tagext.lang.IncludeTag.doStartTag(IncludeTag.java:578)
at coldfusion.filter.CfincludeFilter.invoke(CfincludeFilter.java:65)
at coldfusion.filter.ApplicationFilter.invoke(ApplicationFilter.java:573)
at coldfusion.filter.RequestMonitorFilter.invoke(RequestMonitorFilter.java:43)
at coldfusion.filter.MonitoringFilter.invoke(MonitoringFilter.java:40)
at coldfusion.filter.PathFilter.invoke(PathFilter.java:162)
[snip]