Jennifer Thompson posted a comment on ticket #128

This article resolved the issue on my MacBook (macOS catalina): https://applehint.com/t/awesome-trick-to-fix-dia-diagram-errors-in-mac-os-x-mojave-high-sierra-sierra-el-capitan-yosemite/302 The "export DISPLAY =: 0" command is in an if block in dia.app now, so I just commented out the "if" and "fi", then it worked as outlined in the article.