>Checking what changes between when you can, and when you can't, compile, can be a time-consuming, trial-and-error process.
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>I strongly suspect a file is in use - perhaps you can run some program to "do something" with each of the files involved in the project? Like (a) copying somewhere else, (b) moving somewhere else. Have a recent backup!
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>Expected results: (a) Test the theory that a file is in use. (b) If the answer is yes, the knowledge, which file is in use, may give you a hint what is going on; you may also be able to search for the filename in code, to see if something queer happens with that file.

Thanks for the additional information