UVision5 - object file xxx renamed to yyy bug

I have been working for some time on a project in uVision5. It's been fine until now. Today I added some new targets to differentiate builds for different products, and suddenly I am getting a lot of messages like this, and the project does not build.

*** Using Compiler 'V5.06 update 7 (build 960)', folder: 'C:\Keil_v5\ARM\ARMCC\Bin'
Rebuild target 'BROADCAST'
Note: source file 'C:\Projects\Rubidium\DialogSDK\6.0.14.1114\sdk\platform\arch\boot\system_DA14531.c' - object file renamed from '.\out_BROADCAST\Objects\system_DA14531.o' to '.\out_BROADCAST\Objects\system_d'.
....
error - cannot create command input file '.\out_broadcast\objects\objects\.__i'
Target not created.
Build Time Elapsed:  00:00:00

I can clear the fault by closing uVision, deleting the build folders for the targets and then trying to build. This is making the IDE virtually unusable.

What is causing this?