When running the debugger on High Sierra, xojo fails to delete the debug file after the debug session is closed. This prevents running the debugger again until the debug file is manually deleted.
Maybe this will helps since on my end works well and till know no more this kind of issues.
This issue is common and for High Sierra and Sierra as well. On Sierra even restarting a Mac doesn’t helps but renaming a project filename does.
Best way to handle it so far with what I got is:
In Xojo,in debug settings part select folder location to be some temp folder under your account home folder where your account have full access where Xojo will put temp files.
Restart Xojo and give a try.
Screen shot to solve this: