I get "Build Failed" when using Run

I have a project that “used to work fine”. But after making lots of changes, the currently observed behavior is:

=> Pressing Build, builds the project just fine, creates an executable, and the executable runs just fine.
=> Pressing Run, however, starts building project, flashes messages like “Done Assembling Code”, and then “Build Failed”.

IDE keeps on working fine, but one of the build subwindows seem to crash on Build process before the Run happens …
The “Build Failed” dialog box flashes briefly. So briefly, I did not even see it the first many times I ran it. When I finally did, I could not even grab a screenshot, it comes on and off in an instant, and then it disappears. I do not know Xojo enough, but I do not see and cannot find any diagnostic error/warning messages anywhere. The usual channels for showing errors show blanks.

Something must have changed, as it used to work… Is there a way to somehow see what the problem is? Maybe some logs somehere?

See if there’s a build that failed to get deleted. On Mac it will be next to your project file titled [APP_NAME].debug

Yep that was it. Turns out the Debug folder was locked by another process. Releasing the lock & removing folder restored Run operation.